feat(lotus): layout, settings and reactions over the widget API (cinny #43)

The Lotus host drove these by clicking EC's hidden controls in our DOM and
read screenshare/layout state from it, which only works while the frame is
same-origin and breaks whenever a data-testid changes. New actions:

- toWidget io.lotus.set_layout { layout: "grid" | "spotlight" } → the layout
  switch view model's setLayout.
- toWidget io.lotus.open_settings { open?: boolean } → setSettingsOpen
  (toggle when `open` is omitted).
- toWidget io.lotus.toggle_reactions → a window event the reactions button
  listens for.
- fromWidget io.lotus.controls_state { screensharing, layout } on change; its
  arrival tells the host the fork supports the actions above.

Screensharing stays host-driven for now: getDisplayMedia needs the user's
click to reach this frame (Capability Delegation), which a widget message
doesn't carry.

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

src/button/ReactionToggleButton.tsx

@@ -26,6 +26,7 @@ import { logger } from "matrix-js-sdk/lib/logger";
 import classNames from "classnames";
 
 import { useReactionsSender } from "../reactions/useReactionsSender";
+import { LOTUS_TOGGLE_REACTIONS_EVENT } from "../lotus/lotusControls";
 import styles from "./ReactionToggleButton.module.css";
 import {
   type RaisedHandInfo,
@@ -196,6 +197,15 @@ export function ReactionToggleButton({
     setErrorText(undefined);
   }, [showReactionsMenu]);
 
+  // [cinny #43] The Lotus host's call bar toggles this menu over the widget
+  // API (lotusControls.ts) instead of clicking this button in our DOM.
+  useEffect(() => {
+    const toggle = (): void => setShowReactionsMenu((open) => !open);
+    window.addEventListener(LOTUS_TOGGLE_REACTIONS_EVENT, toggle);
+    return (): void =>
+      window.removeEventListener(LOTUS_TOGGLE_REACTIONS_EVENT, toggle);
+  }, []);
+
   const sendRelation = useCallback(
     async (reaction: ReactionOption) => {
       try {