Add friend chat; replace betting wording (شرط) with entry coins

- Friend-to-friend chat outside the game (ChatScreen) with mock replies,
  per-friend history, unread tracking; chat button on each friend row
- OnlineService + mock + online-store extended with chat (list/get/send/markRead)
- Reframe gambling term: "شرط"/"Stake" -> "سکه ورودی"/"Entry coins";
  free entry labeled رایگان/Free

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

src/app/page.tsx

@@ -10,6 +10,7 @@ import { RoomScreen } from "@/components/screens/RoomScreen";
 import { MatchmakingScreen } from "@/components/screens/MatchmakingScreen";
 import { LeaderboardScreen } from "@/components/screens/LeaderboardScreen";
 import { ShopScreen } from "@/components/screens/ShopScreen";
+import { ChatScreen } from "@/components/screens/ChatScreen";
 import { AuthScreen } from "@/components/screens/AuthScreen";
 import { DailyRewardModal } from "@/components/online/DailyRewardModal";
 import { useSessionStore } from "@/lib/session-store";
@@ -53,6 +54,8 @@ function renderScreen(screen: string) {
       return <LeaderboardScreen />;
     case "shop":
       return <ShopScreen />;
+    case "chat":
+      return <ChatScreen />;
     default:
       return <HomeScreen />;
   }