feat(desktop): add Mac-native zoom shortcuts (Cmd+=/-/0)

[Copilot]

The desktop app had no way to scale the UI; users expect the standard macOS zoom keys to work like in native apps and browsers.

Changes

• useZoom hook (apps/desktop/src/hooks/useZoom.ts)

  • Binds Cmd/Ctrl + = / + (zoom in), Cmd/Ctrl + - (zoom out), Cmd/Ctrl + 0 (reset).
  • Drives zoom through Tauri's built-in getCurrentWebview().setZoom() so the whole webview scales like browser Cmd+/-, rather than via CSS hacks.
  • Clamps to 0.5–2.0 in 0.1 steps (rounded to avoid float drift), persists to localStorage under ccem-zoom-level, restores on mount.
  • keydown listener registered in capture phase so the shortcuts win against inputs, textareas, terminals, and contenteditable surfaces. Combos including Alt are ignored to avoid clashing with other namespaces.

• Capability: added core:webview:allow-set-webview-zoom to apps/desktop/src-tauri/capabilities/default.json — required for the JS setZoom IPC call.

• Wiring: App.tsx calls useZoom() alongside the existing useKeyboardShortcuts. No change to the existing global shortcut map.

// apps/desktop/src/App.tsx
useKeyboardShortcuts(globalShortcuts);
useZoom(); // Cmd+= / Cmd+- / Cmd+0

Notes

• Persistence is intentionally per-webview (localStorage), not part of ~/.ccem/config.json — this is a UI preference, not cross-CLI config.
• An earlier iteration used CSS zoom on documentElement; switched to the native Tauri API after confirming it's the supported path.