Add SRT / VTT / JSON transcript export

[wassgha]

Summary

Adds the ability to export the editor transcript as SRT, WebVTT, or JSON.

• New lib/serializeTranscript.ts mirrors the existing import parser
• SRT / VTT omit deleted words and remap timestamps onto the edited timeline (so captions sync with the exported media), with speaker labels
• JSON writes the full { words: [...] } list (including deleted) for round-trip re-import
• UI: Export dropdown next to Import in the transcript toolbar, plus format buttons in the media Export dialog
• Round-trip coverage in scripts/serialize-transcript-test.ts

Test plan

• npx tsx scripts/serialize-transcript-test.ts
• npx tsx scripts/parse-transcript-test.ts
• npm run lint
• In the editor with a ready transcript, use Export in the transcript toolbar → download SRT / WebVTT / JSON
• Open the top-bar Export dialog → download transcript formats from the Transcript section
• Cut some words, export SRT/VTT, confirm deleted words are gone and times match the shortened edit
• Re-import the JSON export and confirm words/speakers/cuts round-trip