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/GeneralWidgetDriver.ts

@@ -0,0 +1,15 @@
+import { type Capability, WidgetDriver } from 'matrix-widget-api';
+import { filterWidgetCapabilities } from './widgetUtils';
+
+// A minimal, conservative WidgetDriver for general room widgets. It only narrows
+// the capabilities a widget may hold (to a benign display-only subset — see
+// widgetUtils). All data-access methods (sendEvent / readRoomState / sendToDevice
+// / uploads …) are inherited from the base WidgetDriver and are never reached,
+// because the capabilities that would gate them are denied here. A richer,
+// consent-prompt-driven driver is a follow-up.
+export class GeneralWidgetDriver extends WidgetDriver {
+  // eslint-disable-next-line class-methods-use-this
+  public async validateCapabilities(requested: Set<Capability>): Promise<Set<Capability>> {
+    return filterWidgetCapabilities(requested);
+  }
+}