Hide Local (Whisper) mode option when using cloud-only sidecar

- Expose is_cloud_only flag in /api/status response
- Add isCloudOnly to backend store state
- Conditionally hide Local (Whisper) radio button in Settings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app_controller.py

@@ -682,6 +682,7 @@ class AppController:
             "transcription_count": len(self.transcriptions),
             "remote_mode": remote_mode,
             "server_sync_enabled": self.config.get('server_sync.enabled', False),
+            "is_cloud_only": self.is_cloud_only,
         }
 
     def get_audio_devices(self) -> list[dict]: