Show transcription start errors in UI, improve error logging

Start Transcription button now shows the error message when it fails
instead of silently reverting. Common causes:
- Missing PortAudio library on Linux
- Audio device not accessible
- Deepgram connection failure

Also added error details to backend console output and captured
the last error from the Deepgram engine for better diagnostics.

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

backend/app_controller.py

@@ -396,7 +396,14 @@ class AppController:
         try:
             success = self.transcription_engine.start_recording()
             if not success:
-                return False, "Failed to start recording"
+                import logging
+                # Check if there's a recent error in the logger
+                err_detail = getattr(self.transcription_engine, '_last_error', '')
+                msg = f"Failed to start recording"
+                if err_detail:
+                    msg += f": {err_detail}"
+                print(f"ERROR: {msg}")
+                return False, msg
 
             # Start server sync if enabled
             if self.config.get('server_sync.enabled', False):