feat(calls): toast when a room admin turns off screen sharing / camera mid-call (#223)

Re-measured the revoke on the current stack (guard fix + fork lotus.4): the
SFU drops the sharer's tracks and EC's own button, the captured tracks and our
control bar all follow within ~2-4 s — the 30 s lag is gone. What remained was
that the button simply vanished with no explanation, so a call-lifetime hook
now toasts 'Screen sharing was turned off by a room admin.' (or the camera
variant) when a track ends while the room policy forbids it.

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

src/app/components/CallEmbedProvider.tsx

@@ -46,6 +46,7 @@ import { useMatrixClient } from '../hooks/useMatrixClient';
 import { previewRingtone, startRingtone, unlockRingtoneAudio } from '../utils/ringtones';
 import { useCallMembersChange, useCallSession } from '../hooks/useCall';
 import { useCallJoinLeaveSounds } from '../hooks/useCallJoinLeaveSounds';
+import { useCallPolicyRevokedToast } from '../hooks/useCallPolicyRevokedToast';
 import { useCallHotkeys } from '../hooks/useCallHotkeys';
 import { useAfkAutoMute } from '../hooks/useAfkAutoMute';
 import { useCallQuality } from '../hooks/useCallQuality';
@@ -685,6 +686,7 @@ function CallUtils({ embed, joined }: { embed: CallEmbed; joined: boolean }) {
   useCallHotkeys(embed, joined);
   useAfkAutoMute(joined ? embed : undefined);
   useCallJoinLeaveSounds(embed);
+  useCallPolicyRevokedToast(embed, joined);
   useCallThemeSync(embed);
   useCallQuality(embed);
   useCallHangupEvent(