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/desktop.rs

@@ -25,4 +25,8 @@ impl<R: Runtime> NativeBridge<R> {
     pub fn copy_uri_to_path(&self, _payload: CopyURIRequest) -> crate::Result<CopyURIResponse> {
         Err(crate::Error::UnsupportedPlatformError)
     }
+
+    pub fn use_background_audio(&self, _payload: UseBackgroundAudioRequest) -> crate::Result<()> {
+        Err(crate::Error::UnsupportedPlatformError)
+    }
 }