Phase 4: Export to SRT, WebVTT, ASS, plain text, and Markdown

- Implement ExportService using pysubs2 for caption formats (SRT, VTT, ASS)
  and custom formatters for plain text and Markdown
- SRT exports with [Speaker]: prefix, WebVTT with <v Speaker> voice tags,
  ASS with color-coded speaker styles
- Plain text groups by speaker with labels, Markdown adds timestamps
- Add export.start IPC handler and export_transcript Tauri command
- Add export dropdown menu in header (appears after transcription)
- Uses native save dialog for output file selection
- Add pysubs2 dependency
- Tests: 30 Python (6 export tests), 6 Rust, 0 Svelte errors

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

src/lib/services/tauri-bridge.ts

@@ -47,6 +47,27 @@ export interface PipelineResult extends TranscriptionResult {
   num_speakers: number;
 }
 
+export interface ExportResult {
+  output_path: string;
+  format: string;
+}
+
+export async function exportTranscript(
+  segments: Array<{ text: string; start_ms: number; end_ms: number; speaker: string | null }>,
+  speakers: Record<string, string>,
+  format: string,
+  outputPath: string,
+  title?: string,
+): Promise<ExportResult> {
+  return invoke('export_transcript', {
+    segments,
+    speakers,
+    format,
+    outputPath,
+    title,
+  });
+}
+
 export async function runPipeline(
   filePath: string,
   options?: {