Fix cloud sidecar crash on first launch

The cloud sidecar excludes the local Whisper engine module, but on
first launch the config defaults to remote.mode="local" which tries
to import it. Now catches the ImportError gracefully and shows an
error message telling the user to switch to Cloud (Deepgram) mode
in Settings. The API server still starts so Settings is accessible.

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

backend/app_controller.py

@@ -300,8 +300,17 @@ class AppController:
             # Lazy-import heavy local transcription dependencies
             global RealtimeTranscriptionEngine
             if RealtimeTranscriptionEngine is None:
-                from client.transcription_engine_realtime import RealtimeTranscriptionEngine as _RTE
-                RealtimeTranscriptionEngine = _RTE
+                try:
+                    from client.transcription_engine_realtime import RealtimeTranscriptionEngine as _RTE
+                    RealtimeTranscriptionEngine = _RTE
+                except ImportError:
+                    # Cloud-only sidecar -- local engine not available
+                    self._set_state(
+                        AppState.ERROR,
+                        "Local transcription not available in this build. "
+                        "Please switch to Cloud (Deepgram) mode in Settings."
+                    )
+                    return
 
             if self.device_manager:
                 self.device_manager.set_device(device_config)