feat: warn when the local clock is far off the homeserver's (#158)

Incident 2026-09-17: a wrong Windows clock broke calls and media keys while
the server answered 200 to everything, with no hint in the UI.

Measurement needs no extra requests and no CORS-exposed headers: every live
event carries origin_server_ts and unsigned.age (our server's now − ts at
response time), so localTimestamp − origin_server_ts is the skew. Only
RoomEvent.Timeline live events count (cache replays have stale age and are
already flagged liveEvent=false by the SDK); the initial network sync
qualifies, so a wrong clock is flagged within seconds of startup. Median of
the last 5 samples, ≥3 needed; warn at |skew| > 30 s, clear below 15 s.

UI: a banner in the sync-status slot — "Your computer's clock is 14 minutes
ahead of the server. Encrypted messages and voice calls will fail until it is
fixed." with a per-OS How-to-fix hint and Dismiss for 24 h — plus the same
line in the call status bar while in a call. Never auto-corrects anything.

Unit-tested (median, hysteresis, stale-age rejection, wording); verified
headless with Playwright's clock skewed +14 min and −3 h (banner, hint,
in-call line, dismiss) and in sync (nothing shown).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/pages/client/ClientRoot.tsx

@@ -43,6 +43,7 @@ import { AsyncStatus, useAsyncCallback } from '../../hooks/useAsyncCallback';
 import { useSyncState } from '../../hooks/useSyncState';
 import { stopPropagation } from '../../utils/keyboard';
 import { SyncStatus } from './SyncStatus';
+import { ClockSkewBanner } from './ClockSkewBanner';
 import { AuthMetadataProvider } from '../../hooks/useAuthMetadata';
 import { getFallbackSession, removeFallbackSession } from '../../state/sessions';
 import { pushSessionToSW } from '../../../sw-session';
@@ -246,6 +247,7 @@ export function ClientRoot({ children }: ClientRootProps) {
     <AutoDiscovery userId={userId!} baseUrl={baseUrl!}>
       <SpecVersions baseUrl={baseUrl!}>
         {mx && !syncError && <SyncStatus mx={mx} />}
+        {mx && !syncError && <ClockSkewBanner />}
         {loading && <ClientRootOptions mx={mx} />}
         {(loadState.status === AsyncStatus.Error ||
           startState.status === AsyncStatus.Error ||