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/capabilities/default.json

@@ -5,6 +5,14 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
-    "store:default"
+    "store:default",
+    "notification:default",
+    "notification:allow-is-permission-granted",
+    "notification:allow-request-permission",
+    "notification:allow-notify",
+    "autostart:allow-enable",
+    "autostart:allow-disable",
+    "autostart:allow-is-enabled",
+    "window-state:default"
   ]
 }