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>

server/src/Hokm.Server/Game/GameManager.cs

@@ -17,12 +17,11 @@ public sealed class Player
 /// <summary>In-memory matchmaking + room registry. (EF/Postgres persistence is a TODO.)</summary>
 public sealed class GameManager
 {
-    // Real players get priority: wait ~15s for humans before bots fill in. The
-    // exact wait is randomized per ticket (12–18s) so the queue doesn't feel
-    // robotically identical every time.
-    private const int QueueWaitMinMs = 12000;
-    private const int QueueWaitMaxMs = 18000;
-    private int NextQueueWaitMs() => _rng.Next(QueueWaitMinMs, QueueWaitMaxMs + 1);
+    // Real players get priority: wait exactly 15s for humans to join; whoever
+    // hasn't joined the table by then is replaced with a bot when the match forms.
+    // (Mirror of MATCH_QUEUE_WAIT_MS on the client — keep both in sync.)
+    private const int QueueWaitMs = 15000;
+    private int NextQueueWaitMs() => QueueWaitMs;
 
     private static readonly string[] BotNames =
         { "آرش", "کیان", "نیلوفر", "سارا", "رضا", "مهسا", "امیر", "پارسا", "الناز", "بابک" };