feat: stats + chat + frontend pages (Stats, Chat, NavBar)

Backend:
- StatsManager.js: JSON persistence, leaderboard, rate-limit 1/5s
- ChatManager.js: 200-msg buffer, JSON persistence
- index.js: routes GET/POST /stats, /chat/history, /chat/send (Zod validation)
- ArenaRoom.js: chat handler broadcasts to room + persists via ChatManager

Unity:
- StatsTracker.cs: distance, maxSpeed, jumps, bumps, checkpoints, raceTime tracking
- ChatUI.cs: F3 toggle, bottom-right panel, polling 3s, unread badge
- NetworkManager.cs: SendChatMessage() + OnMessage<ChatUI.ChatMessage>(chat)
- CheckpointSystem.cs: RegisterCheckpoint/Finish hooks
- PlayerController.cs: RegisterJump/Bump hooks, physics rebalance, billboard fix
- GameHUD.cs: LocalRaceTimer, SetTotalRounds, OnRoundStart signature fix
- GameManager.cs: spectator cam reconnect fix

Frontend:
- NavBar.jsx: fixed top nav, Accueil/Stats/Chat/Jouer
- App.jsx: page state (home/play/stats/chat) + NavBar
- StatsPage.jsx: 6-tab leaderboard, auto-refresh 30s
- ChatPage.jsx: polling 3s, localStorage name, Enter to send

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

game/Assets/Scripts/Network/NetworkManager.cs

@@ -171,6 +171,10 @@ public class NetworkManager : MonoBehaviour
                 Debug.Log($"[Network] Game over — Winner: {msg.winner}");
                 OnGameEnd?.Invoke(msg.winner);
             });
+            _room.OnMessage<ChatUI.ChatMessage>("chat", msg =>
+            {
+                ChatUI.Instance?.ReceiveChatMessage(msg);
+            });
 
             _room.OnLeave += OnRoomLeave;
             OnConnected?.Invoke();
@@ -206,6 +210,12 @@ public class NetworkManager : MonoBehaviour
             await _room.Send("checkpointReached", new { index });
     }
 
+    public async void SendChatMessage(string text)
+    {
+        if (_room != null && IsConnected)
+            await _room.Send("chat", new { text });
+    }
+
     // ─── State Callbacks ─────────────────────────────────────────────────
 
     private void _OnPhaseChanged(string phase)