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_TODO.md

@@ -105,11 +105,13 @@ Genuine Matrix client-spec / MSC features Lotus does **not** yet implement (audi
 - [x] **Disappearing Messages — MSC1763 `m.room.retention`.** PL-gated room-settings `SettingTile` to set `{ max_lifetime }`; retention badge; a client-side sweep hides/self-redacts own expired events (pattern like the mute-timer restore in `ClientNonUIFeatures.tsx`). True server deletion also wants Synapse `retention:` (LXC 151).
 - [x] **QR Device Verification — reciprocate QR.** Add the QR path beside emoji-SAS in `components/DeviceVerification.tsx`: render with `qrcode.react` (already a dep), scan via `BarcodeDetector` (fallback `jsQR`); uses the SDK `VerificationRequest` QR/reciprocate support.
 
-**Phase C (large — each its own planning session):**
+**Phase C (Room Widgets ✅ 2026-07; Sliding Sync pending — its own session):**
 
-- [ ] **Room Widgets — MSC1236 + widget API.** No general widget UI exists (only the PL entry `im.vector.modular.widgets`; the EC call widget is hardcoded). Read `im.vector.modular.widgets`/`m.widget` state, add an Add/Manage panel + sandboxed iframe renderer via `matrix-widget-api` — **extend the existing EC widget plumbing** (`plugins/call/CallEmbed.ts`). Enables Etherpad/notes/dashboards/integrations.
+- [x] **Room Widgets — MSC1236 + widget API.** No general widget UI exists (only the PL entry `im.vector.modular.widgets`; the EC call widget is hardcoded). Read `im.vector.modular.widgets`/`m.widget` state, add an Add/Manage panel + sandboxed iframe renderer via `matrix-widget-api` — **extend the existing EC widget plumbing** (`plugins/call/CallEmbed.ts`). Enables Etherpad/notes/dashboards/integrations.
 - [ ] **Sliding Sync — MSC3575 / simplified MSC4186.** Lotus is on **legacy full `/sync`** though the server advertises `simplified_msc3575`. matrix-js-sdk ships `SlidingSync`; migration → near-instant cold start + low memory + huge-account scale. Touches the sync/room-list/spaces/unread core — behind a feature flag with a legacy fallback. **Plan separately before touching.**
 
+**Room Widgets v1 follow-ups:** capability-approval consent prompt (let widgets request send/read room events); Jitsi/stickerpicker special types; account-data (user/sticker) widgets; per-widget popout / always-on-screen. Requires the prod CSP `frame-src` widening (done in `matrix/cinny/nginx.conf` → **`nginx -s reload`**) or external widgets are blocked.
+
 **Server-gated / advanced (capture, don't build yet):** QR sign-in for a new device (**MSC4108** rendezvous — needs an HS-side endpoint); dehydrated devices (**MSC3814** — offline key delivery, also helps the E2EE KE cluster); E2EE history key sharing on invite (**MSC3061** `shared_history`, niche); voice broadcast (Element MSC3888, low value — skip).
 
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