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-tauri/src/lib.rs

@@ -3,6 +3,7 @@ pub mod db;
 pub mod sidecar;
 pub mod state;
 
+use commands::export::export_transcript;
 use commands::project::{create_project, get_project, list_projects};
 use commands::transcribe::{run_pipeline, transcribe_file};
 
@@ -17,6 +18,7 @@ pub fn run() {
             list_projects,
             transcribe_file,
             run_pipeline,
+            export_transcript,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");