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/admin/AdminField.tsx

@@ -0,0 +1,81 @@
+import type { ChangeEvent, ReactNode } from "react";
+
+/** 后台文本框：与聊天搜索同款壳层，聚焦只画外框一圈 */
+export default function AdminField({
+  id,
+  label,
+  hint,
+  value,
+  onChange,
+  placeholder,
+  maxLength,
+  as = "input",
+  type = "text",
+  autoComplete,
+  disabled,
+  className = "",
+  minHeight,
+}: {
+  id: string;
+  label: ReactNode;
+  hint?: ReactNode;
+  value: string;
+  onChange: (v: string) => void;
+  placeholder?: string;
+  maxLength?: number;
+  as?: "input" | "textarea";
+  type?: string;
+  autoComplete?: string;
+  disabled?: boolean;
+  className?: string;
+  minHeight?: string;
+}) {
+  const handle = (e: ChangeEvent<HTMLInputElement | HTMLTextAreaElement>) =>
+    onChange(e.target.value);
+
+  return (
+    <div className={className}>
+      <div className="admin-field-head">
+        <label className="field-label" htmlFor={id}>
+          {label}
+        </label>
+        {maxLength != null && (
+          <span className="admin-field-count" aria-hidden>
+            {value.length}/{maxLength}
+          </span>
+        )}
+      </div>
+      {hint != null && hint !== "" && (
+        <p className="meta text-[12.5px] mb-2.5 leading-relaxed">{hint}</p>
+      )}
+      {as === "textarea" ? (
+        <div className="admin-field-shell is-area">
+          <textarea
+            id={id}
+            className="admin-field-input"
+            style={minHeight ? { minHeight } : undefined}
+            maxLength={maxLength}
+            value={value}
+            onChange={handle}
+            placeholder={placeholder}
+            disabled={disabled}
+          />
+        </div>
+      ) : (
+        <div className="admin-field-shell">
+          <input
+            id={id}
+            type={type}
+            className="admin-field-input"
+            maxLength={maxLength}
+            value={value}
+            onChange={handle}
+            placeholder={placeholder}
+            autoComplete={autoComplete}
+            disabled={disabled}
+          />
+        </div>
+      )}
+    </div>
+  );
+}