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/AdminStepper.tsx

@@ -0,0 +1,94 @@
+"use client";
+
+import { Minus, Plus } from "lucide-react";
+import { useEffect, useState, type KeyboardEvent } from "react";
+
+/** 后台数字步进器：0 是合法值；加减与手输都会提交数字 */
+export default function AdminStepper({
+  id,
+  value,
+  onChange,
+  min = 0,
+  max = 999,
+  step = 1,
+  unit,
+  ariaLabel,
+  disabled,
+}: {
+  id?: string;
+  value: number;
+  onChange: (n: number) => void;
+  min?: number;
+  max?: number;
+  step?: number;
+  unit?: string;
+  ariaLabel: string;
+  disabled?: boolean;
+}) {
+  const clamp = (n: number) => Math.min(max, Math.max(min, Math.round(n)));
+  const [text, setText] = useState(() => String(value));
+
+  useEffect(() => {
+    setText(String(value));
+  }, [value]);
+
+  const emit = (n: number) => {
+    const next = clamp(Number.isFinite(n) ? n : min);
+    onChange(next);
+    setText(String(next));
+  };
+
+  const onKeyDown = (e: KeyboardEvent<HTMLInputElement>) => {
+    if (e.key === "ArrowUp") {
+      e.preventDefault();
+      emit(value + step);
+    } else if (e.key === "ArrowDown") {
+      e.preventDefault();
+      emit(value - step);
+    }
+  };
+
+  return (
+    <div className={`admin-stepper${disabled ? " is-disabled" : ""}`}>
+      <button
+        type="button"
+        className="admin-stepper-btn"
+        onClick={() => emit(value - step)}
+        disabled={disabled || value <= min}
+        aria-label="减少"
+        tabIndex={-1}
+      >
+        <Minus size={14} />
+      </button>
+      <input
+        id={id}
+        className="admin-stepper-input"
+        type="text"
+        inputMode="numeric"
+        value={text}
+        aria-label={ariaLabel}
+        disabled={disabled}
+        onFocus={(e) => e.target.select()}
+        onChange={(e) => {
+          const raw = e.target.value.replace(/[^\d]/g, "");
+          setText(raw);
+          if (raw === "") return;
+          onChange(clamp(Number(raw)));
+        }}
+        onBlur={() => emit(text === "" ? min : Number(text))}
+        onKeyDown={onKeyDown}
+      />
+      {unit ? <span className="admin-stepper-unit">{unit}</span> : null}
+      <button
+        type="button"
+        className="admin-stepper-btn"
+        onClick={() => emit(value + step)}
+        disabled={disabled || value >= max}
+        aria-label="增加"
+        tabIndex={-1}
+      >
+        <Plus size={14} />
+      </button>
+    </div>
+  );
+}