fix(ui): also resync an open game on a foreground regain without a stream drop

Closes the residual tail of the previous commit: when the live stream stays
alive across a brief suspend (Telegram/iOS can pause the socket without tearing
it down), an in-game event shed from a full hub buffer is never recovered by
the reconnect refetch (no reconnect) or the open-game poll (it only runs while
the stream is down). Mirror the lobby's focus re-poll: bump app.resync on a
foreground regain that did not drop the stream, and have Game.svelte refetch
the open game once per resync. Also rescues a missed move/game_over after a
suspend. Add a silent-join mock seam + an e2e isolating this path; extend the
ARCHITECTURE §10 fallback note.

ui/src/lib/mock/client.ts

@@ -181,14 +181,22 @@ export class MockGateway implements GatewayClient {
     return { matched: false, game: structuredClone(g.view) };
   }
 
+  // seatOpponent seats a robot in an open game's empty seat and returns the game (null once it is no
+  // longer open). Shared by the live join (which then pushes opponent_joined) and the silent e2e
+  // variant (which omits the push).
+  private seatOpponent(id: string): MockGame | null {
+    const game = this.games.get(id);
+    if (!game || game.view.status !== 'open') return null;
+    game.view.status = 'active';
+    game.view.seats[1] = { seat: 1, accountId: 'robot', displayName: 'Robo', score: 0, hintsUsed: 0, isWinner: false };
+    return game;
+  }
+
   // fillOpponent seats a robot in an open game's empty seat and pushes opponent_joined — the
   // mock of a human or robot taking the seat. A no-op once the game is no longer open.
   private fillOpponent(id: string): void {
-    const game = this.games.get(id);
-    if (!game || game.view.status !== 'open') return;
-    game.view.status = 'active';
-    game.view.seats[1] = { seat: 1, accountId: 'robot', displayName: 'Robo', score: 0, hintsUsed: 0, isWinner: false };
-    this.emit({ kind: 'opponent_joined', gameId: id, state: this.stateOf(game) });
+    const game = this.seatOpponent(id);
+    if (game) this.emit({ kind: 'opponent_joined', gameId: id, state: this.stateOf(game) });
   }
 
   // joinPendingOpponent is the e2e hook (exposed as window.__mock.joinOpponent) to attach the
@@ -198,6 +206,13 @@ export class MockGateway implements GatewayClient {
     if (this.openGameId) this.fillOpponent(this.openGameId);
   }
 
+  // joinPendingOpponentSilently seats the opponent WITHOUT pushing opponent_joined — the mock of an
+  // event shed from a full hub buffer while the stream is alive, so the UI recovers only on a
+  // foreground resync. e2e hook: window.__mock.joinOpponentSilently.
+  joinPendingOpponentSilently(): void {
+    if (this.openGameId) this.seatOpponent(this.openGameId);
+  }
+
   async lobbyPoll(): Promise<MatchResult> {
     if (this.pendingMatch) {
       const g = this.games.get(this.pendingMatch);