Fix backend sync race causing reverted edits

src/app.js

@@ -1767,6 +1767,9 @@ let reconnectTimer = null;
 let backendSyncTimer = null;
 let appVersionPollTimer = null;
 let baselineAppVersion = "";
+let localStateVersion = 0;
+let syncedStateVersion = 0;
+let localStateDirty = false;
 let selectedClassEditId = null;
 let selectedLeaderboardKey = null;
 let selectedGridSessionId = null;
@@ -2134,6 +2137,8 @@ function saveState(options = {}) {
   const persistable = buildPersistableState();
   localStorage.setItem(STORAGE_KEY, JSON.stringify(persistable));
   if (!options.skipBackend) {
+    localStateVersion += 1;
+    localStateDirty = true;
     scheduleBackendSync();
   }
 }
@@ -2675,6 +2680,7 @@ function scheduleBackendSync() {
 }
 
 async function syncStateToBackend() {
+  const syncVersion = localStateVersion;
   try {
     const res = await fetch(`${getBackendUrl()}/api/state`, {
       method: "POST",
@@ -2687,6 +2693,10 @@ async function syncStateToBackend() {
     backend.available = true;
     backend.lastError = "";
     backend.lastSyncAt = new Date().toISOString();
+    syncedStateVersion = Math.max(syncedStateVersion, syncVersion);
+    if (syncVersion === localStateVersion) {
+      localStateDirty = false;
+    }
   } catch (error) {
     backend.available = false;
     backend.lastError = t("error.sync_failed", { msg: error instanceof Error ? error.message : String(error) });
@@ -2740,7 +2750,9 @@ async function checkAppVersion() {
 function startOverlaySync() {
   clearInterval(overlaySyncTimer);
   overlaySyncTimer = setInterval(async () => {
-    await hydrateFromBackend();
+    if (!localStateDirty) {
+      await hydrateFromBackend();
+    }
     if (currentView === "overlay") {
       renderView();
     }