feat: incoming DM call notification with ring tone

When another user starts a call in a DM room, show a fixed-position
notification with caller avatar, name, and Answer/Decline buttons.
A Web Audio API double-pulse ring tone plays until answered, declined,
or the 30-second auto-dismiss fires.

- useIncomingDmCall hook: listens to MatrixRTC SessionStarted/SessionEnded,
  filters DM rooms (encrypted, ≤2 members), auto-dismisses after 30s,
  stops if caller leaves or user joins another call
- IncomingCallNotification component: ring tone, caller info, themed UI
  for LotusGuild Terminal TDS (navy bg, orange border, neon-green Answer)
  and standard Cinny dark/light (CSS vars, folds Button Success/Critical)
- Router.tsx: mount IncomingCallNotification inside CallEmbedProvider

src/app/pages/Router.tsx

@@ -69,6 +69,7 @@ import { CreateSpaceModalRenderer } from '../features/create-space';
 import { SearchModalRenderer } from '../features/search';
 import { getFallbackSession } from '../state/sessions';
 import { CallStatusRenderer } from './CallStatusRenderer';
+import { IncomingCallNotification } from '../components/IncomingCallNotification';
 import { CallEmbedProvider } from '../components/CallEmbedProvider';
 
 export const createRouter = (clientConfig: ClientConfig, screenSize: ScreenSize) => {
@@ -137,6 +138,7 @@ export const createRouter = (clientConfig: ClientConfig, screenSize: ScreenSize)
                           <Outlet />
                         </ClientLayout>
                         <CallStatusRenderer />
+                        <IncomingCallNotification />
                       </CallEmbedProvider>
                       <SearchModalRenderer />
                       <UserRoomProfileRenderer />