feat(06-02): add native desktop features — tray, window-state, autostart, version check

- Add GET /health/version public endpoint to API
- Extend Tauri with 4 plugins: notification, autostart, window-state, store
- Implement close-to-tray with prevent_close + prevent_exit (Pitfall 2)
- Tray menu with Oeffnen/Beenden (German labels)
- Async startup version check against server /health/version
- Generate Tessera-branded icons (yellow T on dark bg, OKLCH palette)
- Update capabilities for notification, autostart, window-state permissions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/desktop/src-tauri/Cargo.toml

@@ -13,5 +13,9 @@ tauri-build = { version = "2", features = [] }
 [dependencies]
 tauri = { version = "2", features = ["tray-icon"] }
 tauri-plugin-store = "2"
+tauri-plugin-notification = "2"
+tauri-plugin-autostart = "2"
+tauri-plugin-window-state = "2"
+reqwest = { version = "0.12", features = ["json"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"