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);

src/app/hooks/useScreenshareNotices.ts

@@ -0,0 +1,48 @@
+import { useEffect } from 'react';
+import { useSetAtom } from 'jotai';
+import { Icons } from 'folds';
+import { CallEmbed } from '../plugins/call';
+import { toastQueueAtom } from '../state/toast';
+
+export type ScreenshareNoticeKind = 'ended' | 'no-frames' | 'alone';
+
+export const describeScreenshareNotice = (kind: ScreenshareNoticeKind): string | undefined => {
+  switch (kind) {
+    case 'ended':
+      return 'Screen sharing stopped — the shared window was closed.';
+    case 'no-frames':
+      return 'Your screen share is showing nothing — the shared window may be minimised or hidden.';
+    case 'alone':
+      return "Still sharing? You've been sharing your screen for 30 minutes with nobody else in the call.";
+    default:
+      return undefined;
+  }
+};
+
+/**
+ * [Gitea EC#39] Toast the fork's io.lotus.screenshare_notice: shared window
+ * closed, no frames for a while, or a long share with nobody watching.
+ */
+export function useScreenshareNotices(embed: CallEmbed): void {
+  const setToast = useSetAtom(toastQueueAtom);
+  useEffect(
+    () =>
+      embed.listenAction<{ data?: { kind?: ScreenshareNoticeKind } }>(
+        'io.lotus.screenshare_notice',
+        (evt) => {
+          const body = describeScreenshareNotice(evt.detail?.data?.kind as ScreenshareNoticeKind);
+          if (!body) return;
+          setToast({
+            id: `screenshare-${evt.detail?.data?.kind}-${Date.now()}`,
+            iconSrc: Icons.ScreenShare,
+            displayName: 'Screen share',
+            body,
+            roomName: embed.room.name ?? 'Voice call',
+            roomId: embed.roomId,
+            sticky: evt.detail?.data?.kind === 'alone',
+          });
+        },
+      ),
+    [embed, setToast],
+  );
+}