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-cta-button.tsx

@@ -0,0 +1,42 @@
+"use client";
+/**
+ * 订阅页主操作按钮
+ *
+ * 视觉规格（与设计稿对齐）：
+ *   - 全宽，52px 高，--radius-xxxl 圆角
+ *   - 粉色渐变（#f96ADE → #f657A0）
+ *   - 白字 16px/600 + 粉色阴影
+ *   - :disabled 灰化
+ */
+import type { ButtonHTMLAttributes, ReactNode } from "react";
+
+import styles from "./subscription-cta-button.module.css";
+
+export interface SubscriptionCtaButtonProps
+  extends ButtonHTMLAttributes<HTMLButtonElement> {
+  children: ReactNode;
+  isLoading?: boolean;
+}
+
+export function SubscriptionCtaButton({
+  children,
+  isLoading = false,
+  className,
+  disabled,
+  ...rest
+}: SubscriptionCtaButtonProps) {
+  const classes = [styles.button, className].filter(Boolean).join(" ");
+  return (
+    <button
+      {...rest}
+      type="button"
+      disabled={disabled || isLoading}
+      className={classes}
+    >
+      <span className={styles.label}>
+        {isLoading ? <span className={styles.spinner} aria-hidden="true" /> : null}
+        {children}
+      </span>
+    </button>
+  );
+}