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
 
 ```