Fix progress feedback, diarization fallback, and dropdown readability

- Stream pipeline progress to frontend via Tauri events so the progress
  overlay updates in real time during transcription/diarization
- Gracefully fall back to transcription-only when diarization fails
  (e.g. pyannote not installed) instead of erroring the whole pipeline
- Add color-scheme: dark to fix native select/option elements rendering
  with unreadable white backgrounds

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/voice_to_notes/services/pipeline.py

@@ -110,27 +110,57 @@ class PipelineService:
                 )
             return result
 
-        # Step 2: Diarize
+        # Step 2: Diarize (with graceful fallback)
         write_message(
             progress_message(request_id, 50, "pipeline", "Starting speaker diarization...")
         )
 
-        diarization = self._diarize_service.diarize(
-            request_id=request_id,
-            file_path=file_path,
-            num_speakers=num_speakers,
-            min_speakers=min_speakers,
-            max_speakers=max_speakers,
-        )
+        diarization = None
+        try:
+            diarization = self._diarize_service.diarize(
+                request_id=request_id,
+                file_path=file_path,
+                num_speakers=num_speakers,
+                min_speakers=min_speakers,
+                max_speakers=max_speakers,
+            )
+        except Exception as e:
+            print(
+                f"[sidecar] Diarization failed, falling back to transcription-only: {e}",
+                file=sys.stderr,
+                flush=True,
+            )
+            write_message(
+                progress_message(
+                    request_id, 80, "pipeline",
+                    "Diarization unavailable, using transcription only..."
+                )
+            )
 
-        # Step 3: Merge
-        write_message(
-            progress_message(request_id, 90, "pipeline", "Merging transcript with speakers...")
-        )
-
-        result = self._merge_results(transcription, diarization.speaker_segments)
-        result.speakers = diarization.speakers
-        result.num_speakers = diarization.num_speakers
+        # Step 3: Merge (or skip if diarization failed)
+        if diarization is not None:
+            write_message(
+                progress_message(request_id, 90, "pipeline", "Merging transcript with speakers...")
+            )
+            result = self._merge_results(transcription, diarization.speaker_segments)
+            result.speakers = diarization.speakers
+            result.num_speakers = diarization.num_speakers
+        else:
+            result = PipelineResult(
+                language=transcription.language,
+                language_probability=transcription.language_probability,
+                duration_ms=transcription.duration_ms,
+            )
+            for seg in transcription.segments:
+                result.segments.append(
+                    PipelineSegment(
+                        text=seg.text,
+                        start_ms=seg.start_ms,
+                        end_ms=seg.end_ms,
+                        speaker=None,
+                        words=seg.words,
+                    )
+                )
 
         elapsed = time.time() - start_time
         print(