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/game/Game.svelte

@@ -21,7 +21,7 @@
   import { shareOrDownloadGcg } from '../lib/share';
   import { getCachedGame, setCachedGame, type CachedGame } from '../lib/gamecache';
   import { patchLobbyGame } from '../lib/lobbycache';
-  import { applyGameOver, applyMoveDelta, type DeltaResult } from '../lib/gamedelta';
+  import { applyGameOver, applyMoveDelta, applyOpponentJoined, type DeltaResult } from '../lib/gamedelta';
   import { telegramClosingConfirmation, telegramHaptic } from '../lib/telegram';
   import {
     BLANK,
@@ -232,9 +232,10 @@
     } else if (e.kind === 'opponent_joined' && e.gameId === id && e.state) {
       // The opponent took the empty seat: adopt the new participants and status in place,
       // leaving the board, rack and any pending placement untouched (no refetch, no flicker).
-      if (view) {
-        view = { ...view, game: { ...view.game, seats: e.state.game.seats, status: e.state.game.status, players: e.state.game.players } };
-        setCachedGame(id, view, moves);
+      const r = applyOpponentJoined(cacheSnapshot(), e.state);
+      if (r) {
+        view = r.view;
+        setCachedGame(id, r.view, r.moves);
       }
     } else if (e.kind === 'notify' && (e.sub === 'friend_added' || e.sub === 'friend_declined')) {
       // A request the player sent was answered: re-derive the in-game "add friend" state.