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/codec.ts

@@ -151,12 +151,13 @@ export function encodeComplaint(gameId: string, word: string, note: string): Uin
   return finish(b, fb.ComplaintRequest.endComplaintRequest(b));
 }
 
-export function encodeEnqueue(variant: Variant, multipleWords: boolean): Uint8Array {
+export function encodeEnqueue(variant: Variant, multipleWords: boolean, vsAi: boolean): Uint8Array {
   const b = new Builder(64);
   const v = b.createString(variant);
   fb.EnqueueRequest.startEnqueueRequest(b);
   fb.EnqueueRequest.addVariant(b, v);
   fb.EnqueueRequest.addMultipleWordsPerTurn(b, multipleWords);
+  fb.EnqueueRequest.addVsAi(b, vsAi);
   return finish(b, fb.EnqueueRequest.endEnqueueRequest(b));
 }
 
@@ -244,6 +245,7 @@ function decodeGameView(g: fb.GameView): GameView {
     moveCount: g.moveCount(),
     endReason: s(g.endReason()),
     lastActivityUnix: Number(g.lastActivityUnix()),
+    vsAi: g.vsAi(),
     seats,
   };
 }
@@ -781,6 +783,7 @@ function emptyGame(): GameView {
     moveCount: 0,
     endReason: '',
     lastActivityUnix: 0,
+    vsAi: false,
     seats: [],
   };
 }