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/MobilePostBar.tsx

@@ -13,10 +13,12 @@ export default function MobilePostBar({
   postId,
   liked,
   likeCount,
+  allowComments = true,
 }: {
   postId: string;
   liked: boolean;
   likeCount: number;
+  allowComments?: boolean;
 }) {
   const [showTop, setShowTop] = useState(false);
 
@@ -53,15 +55,17 @@ export default function MobilePostBar({
           WebkitBackdropFilter: "blur(14px)",
         }}
       >
-        <button
-          type="button"
-          onClick={scrollToComments}
-          className="w-11 h-11 flex items-center justify-center rounded-full transition-colors hover:bg-[var(--accent-soft)] focus-visible:bg-[var(--accent-soft)]"
-          style={{ color: "var(--ink-2)" }}
-          aria-label="跳到评论框"
-        >
-          <MessageSquare size={18} />
-        </button>
+        {allowComments ? (
+          <button
+            type="button"
+            onClick={scrollToComments}
+            className="w-11 h-11 flex items-center justify-center rounded-full transition-colors hover:bg-[var(--accent-soft)] focus-visible:bg-[var(--accent-soft)]"
+            style={{ color: "var(--ink-2)" }}
+            aria-label="跳到评论框"
+          >
+            <MessageSquare size={18} />
+          </button>
+        ) : null}
         {/* 放大点赞按钮的触控区域（样式见 globals.css .j13-bar-like） */}
         <div className="j13-bar-like">
           <LikeButton postId={postId} liked={liked} likeCount={likeCount} variant="compact" />