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/lib/ui-store.ts

@@ -12,6 +12,7 @@ export type Screen =
   | "matchmaking"
   | "leaderboard"
   | "shop"
+  | "chat"
   | "game"; // the table (used for both ai + online)
 
 interface UIStore {