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.

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;
+}