feat(subscription): add VIP subscription page

Add a new /subscription route showing three VIP membership plans (Monthly,
Quarterly, Annual) with a pink→peach banner, per-plan gradient price bars,
a benefits card, a mock activate CTA, and an agreement checkbox.

- New reusable <Checkbox /> in src/app/_components/core for the legal
  agreement toggle
- Hardcoded SUBSCRIPTION_PLANS constant in src/data/constants
- New SubscriptionScreen composed of 7 colocated components (back link,
  user row, banner, plan card, benefits card, CTA button) with CSS
  Modules matching the existing design-token system
- Route registered as protected and reached from /sidebar "Membership"
- Mock CTA: window.alert + router.push(/chat) — no payment calls

Co-Authored-By: Claude <noreply@anthropic.com>

src/app/subscription/components/subscription-benefits-card.tsx

@@ -0,0 +1,41 @@
+"use client";
+/**
+ * 订阅权益卡片
+ *
+ * 视觉规格（与设计稿对齐）：
+ *   - 白色卡片，--radius-xl 圆角
+ *   - 顶部粉色渐变条：居中白色标题
+ *   - 主体：1./2./3./4. 编号列表
+ *   - 行间细分隔线
+ */
+import styles from "./subscription-benefits-card.module.css";
+
+export interface SubscriptionBenefitsCardProps {
+  title: string;
+  items: readonly string[];
+  className?: string;
+}
+
+export function SubscriptionBenefitsCard({
+  title,
+  items,
+  className,
+}: SubscriptionBenefitsCardProps) {
+  return (
+    <section
+      className={[styles.card, className].filter(Boolean).join(" ")}
+    >
+      <header className={styles.header}>
+        <h2 className={styles.title}>{title}</h2>
+      </header>
+      <ol className={styles.list}>
+        {items.map((item, idx) => (
+          <li key={idx} className={styles.item}>
+            <span className={styles.numeral}>{idx + 1}.</span>
+            <span className={styles.text}>{item}</span>
+          </li>
+        ))}
+      </ol>
+    </section>
+  );
+}