feat: add background sync for Matrix client on mobile platforms

- Introduced background sync functionality for the Matrix client, allowing notifications to be received even when the app is backgrounded.
- Added new commands to start, stop, and get the state of background sync.
- Implemented a BackgroundSyncManager to manage sync state and credentials.
- Integrated matrix-sdk for handling Matrix interactions and notifications.
- Updated Cargo.toml to include necessary dependencies for background sync.
- Modified mobile capabilities to include opener permissions.
- Enhanced lib.rs to handle new commands and integrate background sync logic.

src-tauri/capabilities/mobile.json

@@ -7,6 +7,7 @@
   "permissions": [
     "core:default",
     "notification:default",
-    "deep-link:default"
+    "deep-link:default",
+    "opener:default"
   ]
 }