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/components/screens/OnlineLobbyScreen.tsx

@@ -47,7 +47,7 @@ export function OnlineLobbyScreen() {
                 stake === s ? "btn-gold" : "bg-navy-900/70 gold-border text-cream/70 hover:text-cream"
               )}
             >
-              {s === 0 ? t("menu.guest") : s.toLocaleString()}
+              {s === 0 ? t("common.free") : s.toLocaleString()}
             </button>
           ))}
         </div>