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/game/ChatScreen.svelte

@@ -113,4 +113,16 @@
   }
 </script>
 
-<Chat {messages} {myId} {busy} myTurn={isMyTurn} {canSend} {canNudge} {waiting} {nudgeOnCooldown} onsend={sendChat} onnudge={nudge} />
+<Chat
+  {messages}
+  {myId}
+  {busy}
+  myTurn={isMyTurn}
+  {canSend}
+  {canNudge}
+  {waiting}
+  {nudgeOnCooldown}
+  vsAi={!!view && view.game.vsAi}
+  onsend={sendChat}
+  onnudge={nudge}
+/>