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.

docs/UI_DESIGN.md

@@ -53,8 +53,10 @@ Login uses `Screen`.
   stream handler (`lib/app.svelte.ts`) is the one place that runs for every live event regardless of
   the mounted screen, so it owns cross-screen freshness: it upserts the affected game's `GameView`
   into the lobby snapshot (`patchLobbyGame`) on every game event — opponent move, game-over,
-  opponent-joined, and a match/started game (which it also adds) — and advances a not-currently-viewed
-  game's own cache from the move/over delta (`advanceCached`, skipping the game in view, whose mounted
+  opponent-joined, and a match/started game (which it also adds) — patches the lobby's invitations
+  list (`patchLobbyInvitation`: a still-pending invitation upserted, a terminal one removed) on the
+  `invitation` / `invitation_update` notifications, and advances a not-currently-viewed game's own
+  cache from the move/over/joined delta (`advanceCached`, skipping the game in view, whose mounted
   board owns its cache). The game board additionally mirrors the player's **own** move and its own
   `load()` into the lobby snapshot — the two updates no live event carries.
 - **Telegram integration** (`lib/telegram.ts`): inside the Mini App the colour