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/main.py

@@ -9,6 +9,7 @@ from voice_to_notes.ipc.handlers import (
     HandlerRegistry,
     hardware_detect_handler,
     make_diarize_handler,
+    make_export_handler,
     make_pipeline_handler,
     make_transcribe_handler,
     ping_handler,
@@ -25,6 +26,7 @@ def create_registry() -> HandlerRegistry:
     registry.register("hardware.detect", hardware_detect_handler)
     registry.register("diarize.start", make_diarize_handler())
     registry.register("pipeline.start", make_pipeline_handler())
+    registry.register("export.start", make_export_handler())
     return registry