Fix NaN amount corruption and betting race condition

- Validate amount is finite integer before reaching DB (prevents NaN→NULL balance)
- Capture round number before async fetch to avoid stale setMyBet on round change

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

Author: matteomekhail

frontend.tsx

@@ -251,23 +251,27 @@ function BettingPanel({
 
   const handlePlace = async () => {
     if (!selected || amount <= 0) return;
+    const placedForRound = round.num;
     setPlacing(true);
     setError("");
     try {
       const res = await fetch("/api/bet", {
         method: "POST",
         headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ userId, roundNum: round.num, contestant: selected, amount }),
+        body: JSON.stringify({ userId, roundNum: placedForRound, contestant: selected, amount }),
       });
       const data = await res.json();
       if (!res.ok) {
         setError(data.error || "Failed");
         setPlacing(false);
         return;
       }
-      setMyBet({ contestant: selected, amount });
-      onBalanceChange(data.balance);
-      localStorage.setItem("qs_balance", String(data.balance));
+      // Only update if we're still on the same round
+      if (lastRoundRef.current === placedForRound) {
+        setMyBet({ contestant: selected, amount });
+        onBalanceChange(data.balance);
+        localStorage.setItem("qs_balance", String(data.balance));
+      }
     } catch {
       setError("Network error");
     }

server.ts

@@ -594,6 +594,10 @@ const server = Bun.serve<WsData>({
         return new Response(JSON.stringify({ error: "Invalid JSON" }), { status: 400, headers: { "Content-Type": "application/json" } });
       }
 
+      if (!Number.isFinite(amount) || !Number.isInteger(amount) || amount <= 0) {
+        return new Response(JSON.stringify({ error: "Invalid amount" }), { status: 400, headers: { "Content-Type": "application/json" } });
+      }
+
       // Validate round is active and betting is open
       const active = gameState.active;
       if (!active || active.num !== roundNum) {