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>

src/routes/+layout.svelte

@@ -10,6 +10,7 @@
     padding: 0;
     background: #0a0a23;
     color: #e0e0e0;
+    color-scheme: dark;
     font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen,
       Ubuntu, Cantarell, sans-serif;
     overflow: hidden;