Economy: free vs paid games + buy-coins page; friends remove confirmation

- Coins only matter for ranked: free games (vs computer / private friend rooms)
  cost nothing; random ranked requires an entry (stake), gated by balance →
  routes to buy-coins when short
- Buy Coins page (CoinPack/getCoinPacks/buyCoins; mock credits now, real
  Zarinpal/IDPay TODO); TopBar coins → buy; lobby create-room is Free
- Friends: removed instant red ✕ delete; UserMinus → inline confirm before remove

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 { BuyCoinsScreen } from "@/components/screens/BuyCoinsScreen";
 import { ChatScreen } from "@/components/screens/ChatScreen";
 import { NotificationsScreen } from "@/components/screens/NotificationsScreen";
 import { AuthScreen } from "@/components/screens/AuthScreen";
@@ -101,6 +102,8 @@ function renderScreen(screen: string) {
       return <LeaderboardScreen />;
     case "shop":
       return <ShopScreen />;
+    case "buycoins":
+      return <BuyCoinsScreen />;
     case "chat":
       return <ChatScreen />;
     case "notifications":