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/client.ts

@@ -68,7 +68,8 @@ export interface GatewayClient {
   gamesList(): Promise<GameList>;
 
   // --- lobby ---
-  lobbyEnqueue(variant: Variant, multipleWords: boolean): Promise<MatchResult>;
+  /** Start a quick game: human auto-match, or an honest-AI game against a robot when vsAi. */
+  lobbyEnqueue(variant: Variant, multipleWords: boolean, vsAi: boolean): Promise<MatchResult>;
   lobbyPoll(): Promise<MatchResult>;
   /** Leave the auto-match pool (idempotent); a cancelled quick-match must not stay queued. */
   lobbyCancel(): Promise<void>;