feat: honest AI opponent in quick game

New Game's quick game gains an explicit opponent selector — 🤖 AI (default)
or 👤 Random player. AI starts a game seated with a pooled robot that joins
and moves at once: no per-move timeout (a 7-day inactivity loss reusing the
turn-timeout sweeper), chat/nudge disabled, no statistics, the opponent shown
as 🤖 everywhere. The random path (disguised robot) is unchanged.

Driven by one game flag (games.vs_ai), set only on AI-started games so the
disguised path is never revealed; Matchmaker.StartVsAI seats the robot
directly (no open pool); the robot replies event-driven via the game service's
after-commit/after-create hook. Wire: vs_ai on EnqueueRequest + GameView.

ui/src/screens/Lobby.svelte

@@ -63,6 +63,8 @@
   function opponents(g: GameView): string {
     // An auto-match game still waiting for an opponent shows the "searching" placeholder.
     if (g.status === 'open') return t('game.searchingForOpponent');
+    // An honest-AI game shows the robot opponent as 🤖, never its (pooled) name.
+    if (g.vsAi) return '🤖';
     return g.seats
       .filter((s) => s.accountId !== myId)
       .map((s) => s.displayName)