feat(lotus): io.lotus.screenshare_notice — window closed, no frames, sharing alone (#39)

Watches the local screenshare publication and tells the host, once per
share each: the MediaStreamTrack ended (window closed), it has been muted
(no frames — minimised/occluded window) for 15 s, or 30 min of sharing
with nobody else in the call. Detection only; the host renders the
notices. LiveKit already unpublishes an ended share, so nothing lingers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/lotus/lotusActions.ts

@@ -56,6 +56,12 @@ export enum LotusWidgetActions {
   SetAudioOutput = "io.lotus.set_audio_output",
   /** fromWidget: the currently selected output `{ deviceId }` (#119). */
   AudioOutputState = "io.lotus.audio_output_state",
+  /**
+   * fromWidget: local screenshare reminder `{ kind: "ended" | "no-frames" | "alone" }`
+   * — window closed, no frames for 15 s, or 30 min of sharing with nobody
+   * else in the call (#39). Each fires at most once per share.
+   */
+  ScreenshareNotice = "io.lotus.screenshare_notice",
 }
 
 /** toWidget Lotus actions that must be allow-listed in `initializeWidget`. */