feat: 全站评论开关，并统一后台表单控件

关闭评论后前台隐藏评论能力；外观/设置/公告改用 AdminField 与 Stepper。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/CommentsPolicyProvider.tsx

@@ -0,0 +1,52 @@
+"use client";
+
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useRef,
+  useState,
+  type ReactNode,
+} from "react";
+import { useRouter } from "next/navigation";
+import { realtime, RT_SETTINGS_CHANGED, type RtSettingsData } from "@/lib/realtime";
+
+const AllowCommentsContext = createContext(true);
+
+/** 前台是否开放评论（SSR 初值 + 后台开关热更新） */
+export function useAllowComments(): boolean {
+  return useContext(AllowCommentsContext);
+}
+
+/**
+ * 根布局注入：关闭评论后前台立即隐藏评论区/数量；开启则刷新以拉回评论。
+ */
+export default function CommentsPolicyProvider({
+  allowComments,
+  children,
+}: {
+  allowComments: boolean;
+  children: ReactNode;
+}) {
+  const router = useRouter();
+  const [open, setOpen] = useState(allowComments);
+  const openRef = useRef(open);
+  openRef.current = open;
+
+  useEffect(() => {
+    setOpen(allowComments);
+  }, [allowComments]);
+
+  useEffect(() => {
+    return realtime.on<RtSettingsData>(RT_SETTINGS_CHANGED, (data) => {
+      if (typeof data?.allow_comments !== "boolean") return;
+      if (openRef.current === data.allow_comments) return;
+      setOpen(data.allow_comments);
+      router.refresh();
+    });
+  }, [router]);
+
+  return (
+    <AllowCommentsContext.Provider value={open}>{children}</AllowCommentsContext.Provider>
+  );
+}