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/lib/model.ts

@@ -45,6 +45,8 @@ export interface GameView {
   endReason: string;
   /** Lobby sort key: the current turn's start (active) or the finish time (finished), Unix seconds. */
   lastActivityUnix: number;
+  /** true = an honest-AI game: the opponent is shown as 🤖 and chat/nudge/add-friend are disabled. */
+  vsAi: boolean;
   seats: Seat[];
 }