Cloud-first UX: default to Deepgram, gate start button, add room sharing

- Change default transcription mode from local to byok (cloud/Deepgram)
- Move Transcription Mode selector to top of settings for visibility
- Hide local-only settings (model, VAD, timing) when cloud mode selected
- Disable Start button until API key (byok) or login (managed) is configured
- Add room creation and share code flow to Shared Captions section
- Add POST /api/create-room endpoint to Node.js sync server
- Update default sync URL placeholder to caption.shadowdao.com

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.lock

@@ -1881,7 +1881,7 @@ checksum = "92daf443525c4cce67b150400bc2316076100ce0b3686209eb8cf3c31612e6f0"
 
 [[package]]
 name = "local-transcription"
-version = "2.0.8"
+version = "2.0.12"
 dependencies = [
  "bytes",
  "chrono",