feat(guides): autosave structured guide data

hooks/use-autosave.ts

@@ -0,0 +1,66 @@
+"use client";
+
+import { useEffect, useEffectEvent, useState } from "react";
+import { toast } from "sonner";
+
+export type AutosaveState = "saved" | "unsaved" | "saving" | "incomplete" | "error";
+
+interface SaveResult {
+  status: string;
+  message?: string;
+  version?: number;
+}
+
+export function useAutosave<T>({
+  value,
+  valid,
+  initialVersion,
+  persist,
+  delay = 900,
+}: {
+  value: T;
+  valid: boolean;
+  initialVersion: number;
+  persist: (value: T, expectedVersion: number) => Promise<SaveResult>;
+  delay?: number;
+}) {
+  const fingerprint = JSON.stringify(value);
+  const [savedFingerprint, setSavedFingerprint] = useState(fingerprint);
+  const [errorFingerprint, setErrorFingerprint] = useState<string | null>(null);
+  const [saving, setSaving] = useState(false);
+  const [version, setVersion] = useState(initialVersion);
+
+  const save = useEffectEvent(async (snapshot: T, snapshotFingerprint: string) => {
+    setSaving(true);
+    setErrorFingerprint(null);
+    try {
+      const result = await persist(snapshot, version);
+      if (result.status === "error") {
+        setErrorFingerprint(snapshotFingerprint);
+        toast.error(result.message ?? "บันทึกอัตโนมัติไม่สำเร็จ");
+        return;
+      }
+      if (result.version) setVersion(result.version);
+      setSavedFingerprint(snapshotFingerprint);
+    } catch (cause) {
+      setErrorFingerprint(snapshotFingerprint);
+      toast.error(cause instanceof Error ? cause.message : "บันทึกอัตโนมัติไม่สำเร็จ");
+    } finally {
+      setSaving(false);
+    }
+  });
+
+  useEffect(() => {
+    if (!valid || saving || fingerprint === savedFingerprint || fingerprint === errorFingerprint) return;
+    const timer = window.setTimeout(() => {
+      void save(JSON.parse(fingerprint) as T, fingerprint);
+    }, delay);
+    return () => window.clearTimeout(timer);
+  }, [delay, errorFingerprint, fingerprint, savedFingerprint, saving, valid]);
+
+  if (!valid) return "incomplete";
+  if (saving) return "saving";
+  if (fingerprint === errorFingerprint) return "error";
+  if (fingerprint !== savedFingerprint) return "unsaved";
+  return "saved";
+}