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>

python/voice_to_notes/ipc/handlers.py

@@ -117,6 +117,24 @@ def make_pipeline_handler() -> HandlerFunc:
     return handler
 
 
+def make_export_handler() -> HandlerFunc:
+    """Create an export handler."""
+    from voice_to_notes.services.export import ExportService, make_export_request
+
+    service = ExportService()
+
+    def handler(msg: IPCMessage) -> IPCMessage:
+        request = make_export_request(msg.payload)
+        output_path = service.export(request)
+        return IPCMessage(
+            id=msg.id,
+            type="export.result",
+            payload={"output_path": output_path, "format": request.format},
+        )
+
+    return handler
+
+
 def hardware_detect_handler(msg: IPCMessage) -> IPCMessage:
     """Detect hardware capabilities and return recommendations."""
     from voice_to_notes.hardware.detect import detect_hardware