Introduce get_data_root() driven by the COW_DATA_DIR env var so the
packaged desktop build stores config.json, run.log, user data and
WeChat credentials under ~/.cow — surviving app updates and keeping the
app bundle read-only. Source deployments leave COW_DATA_DIR unset and
fall back to the repo root, so existing behavior is unchanged.
- Add a minimal application menu with common items and shortcuts:
New Chat (Cmd+N), Settings (Cmd+,), Reload, etc.
- Add system tray with show window, new chat, and quit; click icon to restore
- Add single-instance lock so relaunching focuses the existing window
- Implement close-to-tray: closing the window hides it; only a real quit destroys it
- Explicitly define Window menu's Close Window bound to Cmd+W / Ctrl+W to reliably trigger hide-to-tray
- Listen for menu-action in the renderer to handle menu-triggered new chat / open settings
