feat(stream): STREAM_BROWSER flag + make toolbar-hide/subtitles broadcast-wide

- Add STREAM_BROWSER (.env) gating screen-share/browser mode. false => the
  /자비스 stream command stays voice + API/MCP only (no Go-Live); true (default)
  => screen share as before. (Browser-driven info retrieval in true mode is a
  follow-up build; the bot has no browser-control tools yet.)
- Make the two test-time fixes broadcast-wide defaults via broadcast-helper.mjs:
  it now also watches every tab for HTML5 fullscreen and toggles Chrome window
  fullscreen so the address bar is hidden for ANY video (xfwm4 won't hide it on
  'f' alone), restoring on exit. Subtitles were already enforced per video.
  scenario.mjs drops its own fullscreen toggle and relies on the helper.
- Revert the test-settings env vars from .env.example (not wanted).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

bot/src/config.ts

@@ -47,6 +47,10 @@ export const config = {
   // STREAM_AUDIO=0 to mute; STREAM_AUDIO_SOURCE overrides the capture source.
   streamAudio: opt("STREAM_AUDIO", "1") !== "0",
   streamAudioSource: opt("STREAM_AUDIO_SOURCE", "@DEFAULT_MONITOR@"),
+  // Screen-share + browser mode. true = the bot may go Live (Go-Live screen
+  // share of the VNC desktop) and drive the on-screen browser for real-time
+  // info. false = no screen share; use voice + API/MCP tools for info only.
+  screenBrowser: opt("STREAM_BROWSER", "true") !== "false",
 
   // novnc backend
   novncUrl: opt("NOVNC_URL", ""),