feat(rooms): Room Widgets (MSC1236 im.vector.modular.widgets)

Phase C.1 of the protocol-gaps roadmap, gate-green (693 tests). Generalizes the
Element Call widget host into a general room-widget feature:
- StateEvent.Widget + widgetsPanelAtom + useRoomWidgets (WidgetParser).
- RoomWidgetView: sandboxed-iframe host via ClientWidgetApi with a conservative
  GeneralWidgetDriver (approves only benign display caps — no room-event
  send/read/to-device). Blocks same-origin widget URLs (sandbox breakout guard).
- WidgetsPanel: list / open / add / remove, PL-gated on im.vector.modular.widgets,
  https + non-same-origin URL validation. Mounted like the media gallery (header
  toggle + 3-way content-panel exclusivity + mobile full-screen overlay).
- Tested URL/capability/id helpers.

Requires the prod CSP frame-src widening (matrix repo) for external widgets.
v1 cuts (capability consent prompt, Jitsi/sticker types, user widgets) noted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/widgets/useRoomWidgets.ts

@@ -0,0 +1,21 @@
+import { Room } from 'matrix-js-sdk';
+import { useMemo } from 'react';
+import { Widget, WidgetParser, IStateEvent } from 'matrix-widget-api';
+import { StateEvent } from '../../../../types/matrix/room';
+import { useRoomState } from '../../../hooks/useRoomState';
+
+/**
+ * All valid `im.vector.modular.widgets` room widgets, reactive on room state.
+ * `WidgetParser` drops empty/removed (`{}`) and malformed entries.
+ */
+export const useRoomWidgets = (room: Room): Widget[] => {
+  const state = useRoomState(room);
+  return useMemo(() => {
+    const widgetEvents = state.get(StateEvent.Widget);
+    if (!widgetEvents) return [];
+    const stateEvents = Array.from(widgetEvents.values()).map(
+      (event) => event.getEffectiveEvent() as unknown as IStateEvent,
+    );
+    return WidgetParser.parseWidgetsFromRoomState(stateEvents);
+  }, [state]);
+};