Fix concurrent checklist sync and consistent actions

static/sync.js

@@ -0,0 +1,26 @@
+(function exposeSync(root, factory) {
+  const api = factory();
+  if (typeof module === "object" && module.exports) module.exports = api;
+  if (root) root.ValleySync = api;
+})(typeof window === "undefined" ? globalThis : window, () => ({
+  mergeSnapshot(localChecks, savingIds, currentRevision, snapshot) {
+    if (!snapshot || !Number.isInteger(snapshot.revision) || snapshot.revision <= currentRevision) {
+      return { applied: false, revision: currentRevision, checks: localChecks };
+    }
+
+    const checks = { ...(snapshot.checks || {}) };
+    for (const itemId of savingIds) {
+      if (Object.prototype.hasOwnProperty.call(localChecks, itemId)) {
+        checks[itemId] = Boolean(localChecks[itemId]);
+      }
+    }
+    return { applied: true, revision: snapshot.revision, checks };
+  },
+
+  confirmWrite(localChecks, currentRevision, result) {
+    return {
+      checks: { ...localChecks, [result.itemId]: Boolean(result.checked) },
+      revision: Math.max(currentRevision, result.revision)
+    };
+  }
+}));