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>

config/default_config.yaml

@@ -42,7 +42,7 @@ transcription:
 
 server_sync:
   enabled: false
-  url: "http://localhost:3000/api/send"
+  url: ""
   room: "default"
   passphrase: ""
   # Font settings are now in the display section (shared for local and server sync)
@@ -69,7 +69,7 @@ web_server:
   host: "127.0.0.1"
 
 remote:
-  mode: local  # local | managed | byok
+  mode: byok  # local | managed | byok
   server_url: ""  # Proxy server URL for managed mode (e.g., wss://your-proxy.com)
   auth_token: ""  # JWT stored after login (managed mode)
   byok_api_key: ""  # Deepgram API key for BYOK mode