feat(widgets): keep a widget on screen while you navigate away (#205)

MSC2931: a widget with m.always_on_screen (auto-granted) can send
set_always_on_screen; the host now answers it, and a widget that asked
keeps running in a small floating window (title, Back to room, close)
when the Widgets panel closes or you switch rooms. Back to room returns
to it with the panel open on that widget, without reloading it.

Moving an iframe reloads it, so widget frames now live in an app-level
layer from the start (widgetSessions.ts) and are positioned over the
panel's placeholder while docked. A widget that did not ask is torn
down as before. One floating widget at a time; it also goes when the
widget withdraws, is removed from the room, or on logout. A permission
request while floating grants nothing extra.

Refs #205

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/pages/Router.tsx

@@ -68,6 +68,7 @@ import { SHARE_PAGE_PATH } from '../../swShare';
 import { RouteError } from './RouteError';
 import { CallStatusRenderer } from './CallStatusRenderer';
 import { CallEmbedProvider } from '../components/CallEmbedProvider';
+import { FloatingWidget } from '../features/room/widgets/FloatingWidget';
 
 const Share = React.lazy(() => import('./client/share/Share').then((m) => ({ default: m.Share })));
 
@@ -197,6 +198,7 @@ export const createRouter = (clientConfig: ClientConfig, screenSize: ScreenSize)
                           <Outlet />
                         </ClientLayout>
                         <CallStatusRenderer />
+                        <FloatingWidget />
                       </CallEmbedProvider>
                       <React.Suspense fallback={null}>
                         <SearchModalRenderer />