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>

LOTUS_TESTING.md

@@ -675,6 +675,8 @@ Run the axe DevTools extension (or Lighthouse → Accessibility) on a room view,
 
 ## Outstanding verification backlog
 
+**Room Widgets (MSC1236, 2026-07 — needs the CSP `frame-src` widening + `nginx -s reload` first):** In a room, the header **Widgets** button (grid icon, desktop) opens a right-side panel. As an admin (PL to modify widgets): **Add Widget** with a name + an https URL (e.g. an Etherpad `https://…` or any embeddable page) → it appears in the list; click it → it renders in a sandboxed iframe in the panel; **Remove** clears it. A non-admin sees the list + can open widgets but has no Add/Remove. Check: a non-https or same-origin URL is rejected on Add with a clear message; the panel is a full-screen overlay on mobile and is mutually exclusive with the Thread/Gallery/Members panels; if a widget stays blank, the prod CSP `frame-src` still needs widening. Widgets get only benign display capabilities (they can't send/read room events in v1).
+
 **QR Device Verification (2026-07):** With two logged-in Lotus sessions (or Lotus + Element), start a device verification. On the **Ready** step you now see your own QR code plus a **"Scan their QR code"** button and a **"Verify with emoji instead"** fallback. Have one device **scan** the other's code (grant camera permission) → the showing device asks you to **Confirm**, and both reach **verified**. Check: emoji-SAS still works unchanged; denying camera shows a graceful "verify with emojis instead" message; a deliberately-wrong scan cancels cleanly. Desktop (WebView2) auto-grants the camera; web needs the Permissions-Policy camera allowance (already set).
 
 **Disappearing Messages (MSC1763 `m.room.retention`, 2026-07):** In Room Settings → General → **Message Retention**, an admin picks Off / 1 Day / 1 Week / 1 Month (non-admins see the buttons disabled). After setting e.g. 1 Day, messages older than a day **vanish from the timeline** for everyone in Lotus (toggle Settings → General → **Show Hidden Events** to reveal them again). Setting back to **Off** restores them. Separately, each user can enable Settings → General → **Enforce Message Retention** (default OFF) → their OWN expired messages then get **permanently redacted** within ~30 s (verify: OTHER people's messages are NEVER redacted by this; only your own). Note true server-side purge also needs Synapse `retention:` configured.