feat(lotus): in-frame screenshare button where the host can't delegate (cinny #43)

Firefox, Safari and WebKitGTK have no Capability Delegation, so the host
can't start a share from its call bar: getDisplayMedia needs the click in
this frame. Until now the host clicked EC's hidden footer button through
the DOM, which only works while the frame is same-origin.

With lotusHostControls + lotusFrameScreenshare the fork now shows EC's own
screenshare button (floating bottom-right, footer still hidden), so the
click is native. controls_state reports frameScreenshare so the host hides
its own button. New toWidget io.lotus.set_frame_screenshare { visible }
lets the host hide it while the room's call policy forbids screensharing
(never while a share is live, so it can still be stopped).

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

src/lotus/LotusFrameScreenshare.tsx

@@ -0,0 +1,41 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type FC } from "react";
+
+import { ShareScreenButton } from "../button";
+import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
+import { useBehavior } from "../useBehavior";
+import {
+  lotusFrameScreenshareEnabled,
+  lotusFrameScreenshareVisible$,
+} from "./lotusControls";
+import styles from "./LotusFrameScreenshare.module.css";
+
+/**
+ * [cinny #43] EC's screenshare button, shown inside the frame when the host
+ * can't start a share for us (`lotusFrameScreenshare`). The rest of EC's
+ * footer stays hidden behind the host's call bar.
+ */
+export const LotusFrameScreenshare: FC<{ vm: CallViewModel }> = ({ vm }) => {
+  const sharing = useBehavior(vm.sharingScreen$);
+  const visible = useBehavior(lotusFrameScreenshareVisible$);
+  if (!lotusFrameScreenshareEnabled() || !vm.toggleScreenSharing) return null;
+  // Hidden by the host's call policy, but never while a share is live: the
+  // user must still be able to stop it.
+  if (!visible && !sharing) return null;
+  return (
+    <div className={styles.floating}>
+      <ShareScreenButton
+        size="md"
+        enabled={sharing}
+        onClick={vm.toggleScreenSharing}
+        data-testid="lotus_frame_screenshare"
+      />
+    </div>
+  );
+};