feat(call): screenshare stays on the call bar in Firefox/Safari via the in-frame prompt (#43)

With a fork that reports `screensharePrompt` (element-call lotus-screenshare-
prompt), the call bar and status bar keep their screenshare button on every
engine. Where the click can't be delegated, starting asks the fork to show
"Share your screen?" inside the call frame (io.lotus.prompt_screenshare)
instead of our own confirm; its Share click starts the share. Stopping works
from the bar as before (no click needed in the frame). Chromium is unchanged.

- useScreenshareMode: hidden (older fork's corner button) | prompt | direct.
- The room's call policy is also pushed to the fork in prompt mode, so the
  prompt never opens where sharing is forbidden.
- Picture-in-picture: the "Return to call" overlay covers the frame; while
  the fork reports the prompt open it lets clicks through to it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/hooks.ts

@@ -75,19 +75,46 @@ export const useCallMicLevel = (callEmbed: CallEmbed | undefined): number => {
 };
 
 /**
- * [Gitea #43] True when EC's own screenshare button is shown inside the call
- * frame (no Capability Delegation in this engine), so the host bar hides its.
+ * [Gitea #43] How the call bar's screenshare button behaves:
+ * - `hidden`: EC's own button is inside the frame (fork lotus.20/21), so ours hides;
+ * - `prompt`: starting asks the fork for its in-frame "Share your screen?"
+ *   (no Capability Delegation in this engine);
+ * - `direct`: our own confirm, then a (delegated) set_screenshare.
  */
-export const useFrameScreenshare = (control: CallControl | undefined): boolean => {
-  const [frame, setFrame] = useState(() => control?.frameScreenshare ?? false);
+export type ScreenshareMode = 'hidden' | 'prompt' | 'direct';
+
+const screenshareMode = (control: CallControl): ScreenshareMode => {
+  if (control.frameScreenshare) return 'hidden';
+  return control.screenshareNeedsPrompt ? 'prompt' : 'direct';
+};
+
+export const useScreenshareMode = (control: CallControl | undefined): ScreenshareMode => {
+  const [mode, setMode] = useState<ScreenshareMode>(() =>
+    control ? screenshareMode(control) : 'direct',
+  );
   useEffect(() => {
     if (!control) {
-      setFrame(false);
+      setMode('direct');
       return undefined;
     }
-    const sync = () => setFrame(control.frameScreenshare);
+    const sync = () => setMode(screenshareMode(control));
     sync();
     return control.onFrameScreenshareChange(sync);
   }, [control]);
-  return frame;
+  return mode;
+};
+
+/** [Gitea #43] The fork's in-frame "Share your screen?" prompt is showing. */
+export const useScreensharePromptOpen = (control: CallControl | undefined): boolean => {
+  const [open, setOpen] = useState(() => control?.screensharePromptOpen ?? false);
+  useEffect(() => {
+    if (!control) {
+      setOpen(false);
+      return undefined;
+    }
+    const sync = () => setOpen(control.screensharePromptOpen);
+    sync();
+    return control.onFrameScreenshareChange(sync);
+  }, [control]);
+  return open;
 };