fix: prevent auto-save race condition during force sync

Skip auto-save when isSyncingRef is true and reset the auto-save timer
at the start of forceSync to prevent concurrent saves from sharing the
same HMAC signature and causing alternating save failures.

apps/web/src/context/GameContext.tsx

@@ -154,10 +154,10 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
         newlyUnlockedRef.current = [];
       }
 
-      // Auto-save every 30 seconds
+      // Auto-save every 30 seconds (skip if a force sync is in-flight to avoid signature collisions)
       if (Date.now() - lastSaveRef.current >= AUTO_SAVE_INTERVAL_MS) {
         lastSaveRef.current = Date.now();
-        if (stateRef.current) {
+        if (stateRef.current && !isSyncingRef.current) {
           void saveGame({
             state: stateRef.current,
             signature: signatureRef.current ?? undefined,
@@ -203,6 +203,7 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
   const forceSync = useCallback(async () => {
     if (!stateRef.current || isSyncingRef.current) return;
     isSyncingRef.current = true;
+    lastSaveRef.current = Date.now(); // push auto-save timer back so it doesn't fire concurrently
     setIsSyncing(true);
     try {
       const response = await saveGame({