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;

src/routes/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
   import { invoke, convertFileSrc } from '@tauri-apps/api/core';
+  import { listen } from '@tauri-apps/api/event';
   import { open, save } from '@tauri-apps/plugin-dialog';
   import WaveformPlayer from '$lib/components/WaveformPlayer.svelte';
   import TranscriptEditor from '$lib/components/TranscriptEditor.svelte';
@@ -89,6 +90,19 @@
     isTranscribing = true;
     transcriptionProgress = 0;
     transcriptionStage = 'Starting...';
+    transcriptionMessage = 'Initializing pipeline...';
+
+    // Listen for progress events from the sidecar
+    const unlisten = await listen<{
+      percent: number;
+      stage: string;
+      message: string;
+    }>('pipeline-progress', (event) => {
+      const { percent, stage, message } = event.payload;
+      if (typeof percent === 'number') transcriptionProgress = percent;
+      if (typeof stage === 'string') transcriptionStage = stage;
+      if (typeof message === 'string') transcriptionMessage = message;
+    });
 
     try {
       const result = await invoke<{
@@ -159,6 +173,7 @@
       console.error('Pipeline failed:', err);
       alert(`Pipeline failed: ${err}`);
     } finally {
+      unlisten();
       isTranscribing = false;
     }
   }