feat: multi-tab sync — save on blur, reload from server on focus

- On blur: immediate save to server (no 30s wait)
- On focus: fetch server save, load if newer than local
- Handles multi-browser scenario (laptop → desktop)

Frontend/src/hooks/useSaveSync.ts

@@ -89,6 +89,36 @@ export function useSaveSync({ getGameState, onLoad, playTimeSeconds }: SaveSyncO
     return () => clearInterval(interval);
   }, [saveToServer, user]);
 
+  // Reload from server on tab focus (multi-tab/multi-browser sync)
+  useEffect(() => {
+    if (!user) return undefined;
+
+    const handleFocus = () => {
+      apiRequest("/save").then((data) => {
+        if (data?.gameState && data.lastSave) {
+          // Only load if server save is newer than our last known save
+          if (!lastSaveRef.current || new Date(data.lastSave) > new Date(lastSaveRef.current)) {
+            onLoad(data.gameState);
+            lastSaveRef.current = data.lastSave;
+            console.info("[SaveSync] Reloaded from server on focus");
+          }
+        }
+      });
+    };
+
+    const handleBlur = () => {
+      saveToServer();
+      console.info("[SaveSync] Saved on blur — other tabs will get latest");
+    };
+
+    window.addEventListener("focus", handleFocus);
+    window.addEventListener("blur", handleBlur);
+    return () => {
+      window.removeEventListener("focus", handleFocus);
+      window.removeEventListener("blur", handleBlur);
+    };
+  }, [user, onLoad]);
+
   // Save on page unload
   useEffect(() => {
     const handleUnload = () => {