[ttncode]

Adds a Tauri 2 desktop shell in desktop/ around the existing static build.

• Compact window, tray, hide on close, start with Windows (hidden), global shortcut Ctrl+Alt+B.
• Session-end notification comes from a Rust timer, so it fires while the window is hidden. Clicking it opens the app.
• Updates through GitHub Releases; never installs while a session runs.
• Browser keys disabled so in-app shortcuts win. CSP allows Boost's YouTube and Spotify embeds.
• CI: Windows job with unit, Rust and end-to-end tests. Tag push builds the release.
• Extension and web build keep shipping. Desktop app starts with empty data.