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.

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")
     }