feat(lobby): keep lobby/game caches fresh from any screen + invitation delta channel

Builds on the cross-screen cache work: the global stream handler now keeps both
caches current no matter which screen is mounted, and invitations become a live
delta channel so the lobby's invitations list is fresh from any screen too.

Client (boundary already started):
- advanceCached now also folds opponent_joined into a not-currently-viewed game's
  cache via a new pure reducer applyOpponentJoined (extracted and reused by the
  mounted game board), so opening an open game that filled while you were elsewhere
  is flash-free.
- patchLobbyInvitation upserts a still-pending invitation and removes a terminal
  one (started/declined/cancelled/expired); the global notify handler calls it on
  the invitation / invitation_update sub-kinds.

Invitations delta channel (no wire/gateway/connector change — the notification
already carries the full invitation with id/status/game_id end to end):
- notify: a new in-app-only NotifyInvitationUpdate sub + NotificationInvitationUpdate
  constructor (shares encoding with NotificationInvitation). The Telegram connector
  renders no message for it, so a decline/cancel never becomes an out-of-app push.
- lobby: emit the changed invitation to every participant on respond (accept/decline),
  on the final accept's game start, and on cancel — so each participant's lobby patches
  its list in place. The authoritative list holds only pending invitations, so the
  client's pending-vs-terminal rule matches it exactly.

Tests: applyOpponentJoined + patchLobbyInvitation unit tests (TDD), the
NotificationInvitationUpdate encoding unit test, and integration assertions that
decline/cancel/accept publish invitation_update to every participant. Full local
suite green (backend unit+integration, UI check/unit/build/bundle/e2e). Docs:
ARCHITECTURE §10 (notify catalog + in-app-only note) and UI_DESIGN updated.

ui/src/lib/gamedelta.ts

@@ -82,5 +82,19 @@ export function advanceCached(cached: CachedGame | undefined, e: PushEvent): Cac
     return applyMoveDelta(cached, { move: e.move, game: e.game, bagLen: e.bagLen }).cache;
   }
   if (e.kind === 'game_over') return applyGameOver(cached, e.game).cache;
+  if (e.kind === 'opponent_joined' && e.state) return applyOpponentJoined(cached, e.state);
   return undefined;
 }
+
+/**
+ * applyOpponentJoined adopts an opponent_joined event into a cached open game: it overlays the
+ * refreshed seats, status and player count from the event's StateView onto the cached game, leaving
+ * the board, the starter's rack and everything else intact (the join changes who is seated, not the
+ * position). It returns undefined when the game is not cached (the next open cold-loads it). It backs
+ * both the mounted game board's in-place update and the global handler's off-screen cache warming.
+ */
+export function applyOpponentJoined(cached: CachedGame | undefined, state: StateView): CachedGame | undefined {
+  if (!cached) return undefined;
+  const game: GameView = { ...cached.view.game, seats: state.game.seats, status: state.game.status, players: state.game.players };
+  return { view: { ...cached.view, game }, moves: cached.moves };
+}