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/components/CallEmbedProvider.tsx

@@ -38,7 +38,7 @@ import {
 } from '../hooks/useCallEmbed';
 import { callChatAtom, callEmbedAtom } from '../state/callEmbed';
 import { toastQueueAtom } from '../state/toast';
-import { CallEmbed, useCallControlState } from '../plugins/call';
+import { CallEmbed, useCallControlState, useScreensharePromptOpen } from '../plugins/call';
 import { hangupCallAndWait } from '../plugins/call/hangup';
 import { useSelectedRoom } from '../hooks/router/useSelectedRoom';
 import { ScreenSize, useScreenSizeContext } from '../hooks/useScreenSize';
@@ -882,6 +882,7 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
   const callActive = callEmbed && joined;
   const callVisible = inCallRoom && callActive && !chatOnlyView;
   const pipMode = callActive && !inCallRoom;
+  const pipPromptOpen = useScreensharePromptOpen(callEmbed?.control);
   const { navigateRoom } = useRoomNavigate();
 
   const { screenshare: pipScreenshare } = useCallControlState(callEmbed?.control);
@@ -1363,6 +1364,9 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
                 zIndex: 1,
                 background: 'transparent',
                 cursor: 'grab',
+                // [Gitea #43] The call's "Share your screen?" prompt is drawn
+                // inside the frame: let the click reach its Share button.
+                pointerEvents: pipPromptOpen ? 'none' : undefined,
                 display: 'flex',
                 alignItems: 'flex-start',
                 justifyContent: 'flex-end',