platform: fix macOS data path + add get_os_info + frontend osInfo helper

CROSS-PLATFORM AUDIT:
- Linux x86_64 (Fedora 43, KDE Wayland): HIGH confidence (the dev target)
- Linux other distros / X11: MEDIUM (tested patterns, untested distros)
- Windows 10/11: LOW — theoretically supported via Tauri + CPAL + whisper.cpp
  + tauri-plugin-global-shortcut (the custom evdev hotkey is no-op on
  Windows); has had zero hands-on testing
- macOS Apple Silicon / Intel: LOW — same; macOS Info.plist needs
  NSMicrophoneUsageDescription for the bundled app, and the path bug
  fixed in this commit

REAL BUG FIXED — macOS app_data_dir was Unix-style:
crates/storage/src/file_storage.rs::app_data_dir() previously fell into
the "Unix" branch on macOS and wrote to ~/.kon/, which violates Apple
guidelines and confuses install/uninstall tooling. Now correctly:
- Windows: %LOCALAPPDATA%/kon (unchanged)
- macOS: ~/Library/Application Support/Kon/
- Linux: $XDG_DATA_HOME/kon or ~/.local/share/kon (XDG Base Directory),
         with legacy ~/.kon/ fallback so existing installs keep working
- Other Unix: ~/.kon/ (unchanged)

OS DETECTION LAYER:
- New get_os_info Tauri command in commands/diagnostics.rs returns:
  {os, arch, family, usesCmd, isWayland, customHotkeyBackend,
   primaryModifierLabel}
- New src/lib/utils/osInfo.js helper:
  - async loadOsInfo() warms a cache, called once at root layout mount
  - then isMac() / isWindows() / isLinux() / modKeyLabel() / isWayland()
    are synchronous
  - browser-preview fallback reads navigator.platform
- Lets components localise hotkey labels (Cmd vs Ctrl), file-picker
  copy ("Open Finder" vs "Open Explorer"), etc, without recomputing in
  every place.

cargo check -p kon-storage clean. Updated HANDOVER-2026-04-17.md with
a per-platform confidence table.

HANDOVER-2026-04-17.md

@@ -187,6 +187,31 @@ env GDK_BACKEND=x11 WINIT_UNIX_BACKEND=x11 \
   short term. The eventual destination is SQLite-only with localStorage
   as a transparent cache.
 
+## Cross-platform status (audited 2026/04/17)
+
+| Platform | Build | Run | Polish | Confidence |
+|---|---|---|---|---|
+| **Linux x86_64 (Fedora 43, KDE Wayland)** | ✓ | ✓ | The everyday dev target | **HIGH** — this is what the sprint was developed against |
+| **Linux x86_64 (other distros, X11)** | Should work | Should work | Wayland self-relaunch is no-op on X11 sessions, evdev hotkeys may need user added to `input` group | **MEDIUM** — tested patterns, untested distros |
+| **Windows 10/11 x86_64** | Untested but should compile (CPAL + Tauri + whisper.cpp all support it) | Untested | Custom evdev hotkeys are no-op; falls back to Tauri's global-shortcut plugin which works on Windows | **LOW** — theoretically supported, has had zero hands-on testing |
+| **macOS aarch64 (Apple Silicon)** | Untested | Untested | Path bug fixed this commit (now uses `~/Library/Application Support/Kon/`); Info.plist needs `NSMicrophoneUsageDescription` for the app bundle | **LOW** — theoretically supported, has had zero hands-on testing |
+| **macOS x86_64 (Intel)** | Same as Apple Silicon | Same | Same | **LOW** |
+
+**For the friends beta on Linux only**, this matters not at all. For a future Windows or macOS build, expect to spend a focused day or two debugging:
+
+- Tauri config: `bundle.macOS.entitlements`, `bundle.windows.signingIdentity`
+- macOS code-signing + notarisation (real money: ~£75/yr Apple developer account)
+- Windows code-signing certificate (~£100-300/yr) or accept SmartScreen warning
+- whisper-rs-sys build dependencies per OS (cmake on all; Visual Studio Build Tools on Windows)
+- macOS-specific Info.plist keys for microphone permission
+
+### What this sprint added on the cross-platform front
+
+- `crates/storage/src/file_storage.rs::app_data_dir()` now correctly handles macOS (`~/Library/Application Support/Kon/`) and Linux (XDG-aware, `~/.local/share/kon`, with legacy `~/.kon` fallback for existing installs). Windows path unchanged.
+- New Tauri command `get_os_info` returns `{os, arch, family, usesCmd, isWayland, customHotkeyBackend, primaryModifierLabel}` so the frontend can adapt UI strings (Cmd vs Ctrl labels, "Open Finder" vs "Open Explorer", etc).
+- New `src/lib/utils/osInfo.js` helper: async `loadOsInfo()` warms a cache, then `isMac() / isWindows() / isLinux() / modKeyLabel() / isWayland()` are synchronous. Eagerly loaded at app startup in the root layout.
+- Falls back gracefully in browser-preview mode by reading `navigator.platform`.
+
 ## Files changed this sprint
 
 ```

crates/storage/src/file_storage.rs

@@ -1,17 +1,55 @@
 use std::path::PathBuf;
 
-/// Resolve the app data directory.
-/// Windows: %LOCALAPPDATA%/kon
-/// Unix: ~/.kon
+/// Resolve the per-user app data directory, following each OS's convention:
+///
+/// - Windows: `%LOCALAPPDATA%\kon\`           e.g. `C:\Users\Jake\AppData\Local\kon`
+/// - macOS:   `~/Library/Application Support/Kon/`
+/// - Linux:   `$XDG_DATA_HOME/kon` or `~/.local/share/kon` (XDG Base Directory),
+///            with a fallback to the legacy `~/.kon/` if it already exists, so
+///            existing installs keep working.
+/// - Other Unix: `~/.kon/`
 ///
 /// TODO: Consolidate with `crates/transcription/src/model_manager.rs::dirs_path()`
 /// into a shared helper in `crates/core/` to avoid duplicating platform-specific
 /// path logic across crates.
 pub fn app_data_dir() -> PathBuf {
-    if cfg!(target_os = "windows") {
+    #[cfg(target_os = "windows")]
+    {
         let local_app_data = std::env::var("LOCALAPPDATA").unwrap_or_else(|_| ".".to_string());
-        PathBuf::from(local_app_data).join("kon")
-    } else {
+        return PathBuf::from(local_app_data).join("kon");
+    }
+
+    #[cfg(target_os = "macos")]
+    {
+        let home = std::env::var("HOME").unwrap_or_else(|_| "/tmp".to_string());
+        return PathBuf::from(home)
+            .join("Library")
+            .join("Application Support")
+            .join("Kon");
+    }
+
+    #[cfg(target_os = "linux")]
+    {
+        let home = std::env::var("HOME").unwrap_or_else(|_| "/tmp".to_string());
+
+        // Honour the legacy ~/.kon/ if it exists on disk so existing
+        // installs are not orphaned. New installs follow XDG.
+        let legacy = PathBuf::from(&home).join(".kon");
+        if legacy.exists() {
+            return legacy;
+        }
+
+        // XDG Base Directory: $XDG_DATA_HOME/kon or default ~/.local/share/kon
+        if let Ok(xdg) = std::env::var("XDG_DATA_HOME") {
+            if !xdg.is_empty() {
+                return PathBuf::from(xdg).join("kon");
+            }
+        }
+        return PathBuf::from(home).join(".local").join("share").join("kon");
+    }
+
+    #[cfg(not(any(target_os = "windows", target_os = "macos", target_os = "linux")))]
+    {
         let home = std::env::var("HOME").unwrap_or_else(|_| "/tmp".to_string());
         PathBuf::from(home).join(".kon")
     }

src-tauri/src/commands/diagnostics.rs

@@ -344,6 +344,72 @@ fn read_tail(path: &PathBuf, n_bytes: u64) -> std::io::Result<String> {
     Ok(String::from_utf8_lossy(&buf).to_string())
 }
 
+/// Cross-platform OS info exposed to the frontend so the UI can adapt:
+/// hotkey labels (Cmd vs Ctrl), file-picker text, "open file location"
+/// terminology, etc. Returned on demand via the `get_os_info` Tauri
+/// command rather than recomputed in JS so we have a single source of
+/// truth in the Rust layer.
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct OsInfo {
+    /// "windows" | "macos" | "linux" | "ios" | "android" | other
+    pub os: String,
+    /// "x86_64" | "aarch64" | other
+    pub arch: String,
+    /// Human-readable platform family for tooltips: "Windows", "macOS",
+    /// "Linux"
+    pub family: String,
+    /// True if the platform uses the Cmd modifier rather than Ctrl as
+    /// the primary accelerator (macOS).
+    pub uses_cmd: bool,
+    /// True if the user is currently in a Wayland session (Linux only).
+    /// Useful for status display; the actual workaround is automatic.
+    pub is_wayland: bool,
+    /// True if the custom evdev hotkey listener is the primary backend
+    /// (Linux only). Otherwise the Tauri global-shortcut plugin is used.
+    pub custom_hotkey_backend: bool,
+    /// Convenience: a localised label for the primary modifier
+    /// ("Cmd" on macOS, "Ctrl" elsewhere).
+    pub primary_modifier_label: String,
+}
+
+#[tauri::command]
+pub fn get_os_info() -> OsInfo {
+    let os = std::env::consts::OS.to_string();
+    let arch = std::env::consts::ARCH.to_string();
+    let family = match os.as_str() {
+        "windows" => "Windows",
+        "macos" => "macOS",
+        "linux" => "Linux",
+        "ios" => "iOS",
+        "android" => "Android",
+        _ => "Unknown",
+    }
+    .to_string();
+    let uses_cmd = os == "macos";
+    let primary_modifier_label = if uses_cmd { "Cmd" } else { "Ctrl" }.to_string();
+
+    let is_wayland = if os == "linux" {
+        std::env::var("XDG_SESSION_TYPE")
+            .map(|v| v.eq_ignore_ascii_case("wayland"))
+            .unwrap_or(false)
+    } else {
+        false
+    };
+
+    let custom_hotkey_backend = os == "linux";
+
+    OsInfo {
+        os,
+        arch,
+        family,
+        uses_cmd,
+        is_wayland,
+        custom_hotkey_backend,
+        primary_modifier_label,
+    }
+}
+
 /// Write the report to a file and return the path. Lets the user save it
 /// somewhere they can attach to an email.
 #[tauri::command]

src-tauri/src/lib.rs

@@ -252,6 +252,7 @@ pub fn run() {
             commands::diagnostics::list_crash_files,
             commands::diagnostics::generate_diagnostic_report,
             commands::diagnostics::save_diagnostic_report,
+            commands::diagnostics::get_os_info,
             commands::live::start_live_transcription_session,
             commands::live::stop_live_transcription_session,
             // Windows

src/lib/utils/osInfo.js

@@ -0,0 +1,100 @@
+// Cross-platform OS info for the Svelte frontend. Single async fetch,
+// cached in memory, exposes synchronous helpers after first await.
+//
+// Backed by the `get_os_info` Tauri command (commands/diagnostics.rs).
+// In browser-preview mode (no Tauri runtime), falls back to a sensible
+// default based on navigator.platform.
+//
+// Usage:
+//   import { loadOsInfo, osInfo, modKeyLabel, isMac } from '$lib/utils/osInfo.js';
+//   await loadOsInfo();          // call once at app startup
+//   modKeyLabel();               // "Cmd" on macOS, "Ctrl" elsewhere
+//   if (isMac()) { ... }
+//
+// The store version (osInfo) is reactive — Svelte components can read it
+// directly with `$osInfo` once Svelte 5 runes are everywhere, or via
+// destructuring on a non-reactive read.
+
+import { invoke } from '@tauri-apps/api/core';
+import { hasTauriRuntime } from './runtime.js';
+
+let cached = null;
+
+const FALLBACK_BROWSER_INFO = {
+  os: detectBrowserOs(),
+  arch: 'unknown',
+  family: detectBrowserFamily(),
+  usesCmd: detectBrowserOs() === 'macos',
+  isWayland: false,
+  customHotkeyBackend: false,
+  primaryModifierLabel: detectBrowserOs() === 'macos' ? 'Cmd' : 'Ctrl',
+};
+
+function detectBrowserOs() {
+  if (typeof navigator === 'undefined') return 'unknown';
+  const ua = (navigator.userAgent || '').toLowerCase();
+  const platform = (navigator.platform || '').toLowerCase();
+  if (platform.includes('mac') || ua.includes('mac os')) return 'macos';
+  if (platform.includes('win') || ua.includes('windows')) return 'windows';
+  if (platform.includes('linux') || ua.includes('linux')) return 'linux';
+  return 'unknown';
+}
+
+function detectBrowserFamily() {
+  switch (detectBrowserOs()) {
+    case 'macos': return 'macOS';
+    case 'windows': return 'Windows';
+    case 'linux': return 'Linux';
+    default: return 'Unknown';
+  }
+}
+
+/** Fetch OS info from the Tauri backend. Idempotent — call multiple times,
+ *  only the first does the round-trip. Browser-preview mode returns the
+ *  navigator-detected fallback. */
+export async function loadOsInfo() {
+  if (cached) return cached;
+  if (!hasTauriRuntime()) {
+    cached = FALLBACK_BROWSER_INFO;
+    return cached;
+  }
+  try {
+    cached = await invoke('get_os_info');
+  } catch (err) {
+    console.warn('loadOsInfo: get_os_info failed, using browser fallback', err);
+    cached = FALLBACK_BROWSER_INFO;
+  }
+  return cached;
+}
+
+/** Synchronous accessor. Returns null if loadOsInfo has not been awaited
+ *  yet. Components that need the value at first render should await
+ *  loadOsInfo() in onMount before reading. */
+export function osInfo() {
+  return cached;
+}
+
+export function isMac() {
+  return cached?.os === 'macos';
+}
+
+export function isWindows() {
+  return cached?.os === 'windows';
+}
+
+export function isLinux() {
+  return cached?.os === 'linux';
+}
+
+/** Localised label for the primary keyboard modifier. "Cmd" on macOS,
+ *  "Ctrl" everywhere else. Use in hotkey display strings:
+ *      `${modKeyLabel()}+Shift+R` */
+export function modKeyLabel() {
+  return cached?.primaryModifierLabel ?? 'Ctrl';
+}
+
+/** True if the current Linux session is Wayland. False on Windows/macOS.
+ *  Useful for Settings → Audio "PipeWire detected" status display. */
+export function isWayland() {
+  return !!cached?.isWayland;
+}

src/routes/+layout.svelte

@@ -7,6 +7,7 @@
   import Titlebar from "$lib/components/Titlebar.svelte";
   import ToastViewport from "$lib/components/ToastViewport.svelte";
   import { hasTauriRuntime } from "$lib/utils/runtime.js";
+  import { loadOsInfo } from "$lib/utils/osInfo.js";
   import { page, settings, saveSettings } from "$lib/stores/page.svelte.js";
   import { getPreferences, updatePreferences } from "$lib/stores/preferences.svelte.js";
   import { hasTauriRuntime } from "$lib/utils/runtime.js";
@@ -203,6 +204,10 @@
     // Diagnostics: capture every uncaught frontend error to error_log.
     installGlobalErrorCapture();
 
+    // OS detection: warm the cache so components can use modKeyLabel() /
+    // isMac() / isWayland() synchronously after first render.
+    loadOsInfo().catch(() => { /* fallback already populated */ });
+
     if (!tauriRuntimeAvailable) {
       hotkeyBackend = "unavailable";
       return;