feat(calls): toast the fork's screenshare reminders (EC #39)

io.lotus.screenshare_notice → "Screen sharing stopped — the shared window
was closed.", "Your screen share is showing nothing — the shared window
may be minimised or hidden.", and a sticky "Still sharing?" after 30 min
with nobody else in the call (fork ≥ 0.25.0-lotus.12).

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

src/app/components/CallEmbedProvider.tsx

@@ -50,6 +50,7 @@ import { useCallPolicyRevokedToast } from '../hooks/useCallPolicyRevokedToast';
 import { useCallEndedToast } from '../hooks/useCallEndedToast';
 import { useCallRejoin } from '../hooks/useCallRejoin';
 import { usePttHaptics } from '../hooks/usePttHaptics';
+import { useScreenshareNotices } from '../hooks/useScreenshareNotices';
 import { useCallAnnouncements } from '../hooks/useCallAnnouncements';
 import { useMutedTalkWarning } from '../hooks/useMutedTalkWarning';
 import { callAnnouncementAtom } from '../state/callAnnouncement';
@@ -748,6 +749,7 @@ function CallUtils({ embed, joined }: { embed: CallEmbed; joined: boolean }) {
   useCallJoinLeaveSounds(embed);
   useCallPolicyRevokedToast(embed, joined);
   useCallEndedToast(embed);
+  useScreenshareNotices(embed);
   usePttHaptics();
   useCallAnnouncements(embed, joined);
   useMutedTalkWarning(embed, joined);