matchmaking: deterministic 15s wait before bots fill empty seats

Both the mock and the .NET server already waited then bot-filled, but used a
random 12-18s window. Make it exactly 15s on both sides so the rule is clear:
wait 15s for real online players to join, then replace any unfilled seats with
bots and start.

- client: new MATCH_QUEUE_WAIT_MS = 15000 in gamification.ts; mock beginSearch
  uses it instead of randInt(12000,18000).
- server: GameManager QueueWaitMs = 15000 (was randomized 12-18s per ticket).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/online/gamification.ts

@@ -178,6 +178,13 @@ export const SPEED_TURN_MS = 5000;
 /** Speed mode races to fewer points so a match is over fast. */
 export const SPEED_TARGET_SCORE = 5;
 
+/**
+ * How long matchmaking waits for real online players before filling the
+ * remaining seats with bots and starting the match. (Mirror of the server's
+ * QueueWaitMs in GameManager.cs — keep both in sync.)
+ */
+export const MATCH_QUEUE_WAIT_MS = 15000;
+
 export function turnMsForStake(stake: number, speed = false): number {
   if (speed) return SPEED_TURN_MS;
   if (stake >= 1000) return 7000;