fix: tts now works in background in iOS, closes #547 (#822)

To enable background playback in Android, go to Settings > Apps & Notifications > Readest > Battery > Battery Optimization, and disable battery optimization for Readest.

apps/readest-app/src-tauri/plugins/tauri-plugin-native-bridge/src/lib.rs

@@ -40,6 +40,7 @@ pub fn init<R: Runtime>() -> TauriPlugin<R> {
             commands::auth_with_safari,
             commands::auth_with_custom_tab,
             commands::copy_uri_to_path,
+            commands::use_background_audio,
         ])
         .setup(|app, api| {
             #[cfg(mobile)]