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>

client/deepgram_transcription.py

@@ -167,6 +167,8 @@ class DeepgramTranscriptionEngine:
             self._start_audio_stream()
         except Exception as exc:
             logger.error("Failed to open audio stream: %s", exc)
+            print(f"ERROR: Failed to open audio stream: {exc}")
+            self._last_error = f"Audio stream error: {exc}"
             self._is_recording = False
             self._stop_event.set()
             return False