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/data/constants/subscription-plans.ts

@@ -0,0 +1,49 @@
+/**
+ * VIP 订阅套餐常量
+ *
+ * Mock 阶段硬编码，后续可改为从 `ApiPath.paymentProducts` 拉取。
+ *
+ * 原始 Dart: lib/ui/subscription/...
+ */
+export type SubscriptionPlanId = "monthly" | "quarterly" | "annual";
+
+export interface SubscriptionPlan {
+  id: SubscriptionPlanId;
+  /** 卡片顶部名称，例如 "Monthly" */
+  name: string;
+  /** 当前价格 */
+  price: number;
+  /** 划线原价 */
+  originalPrice: number;
+  /** 每日折算价字符串，e.g. "$0.66/day" */
+  perDay: string;
+  /** 是否高亮（默认推荐：月付） */
+  highlight: boolean;
+}
+
+export const SUBSCRIPTION_PLANS: readonly SubscriptionPlan[] = [
+  {
+    id: "monthly",
+    name: "Monthly",
+    price: 19.9,
+    originalPrice: 24.9,
+    perDay: "$0.66/day",
+    highlight: true,
+  },
+  {
+    id: "quarterly",
+    name: "Quarterly",
+    price: 49.9,
+    originalPrice: 59.9,
+    perDay: "$0.55/day",
+    highlight: false,
+  },
+  {
+    id: "annual",
+    name: "Annual",
+    price: 169.9,
+    originalPrice: 199.9,
+    perDay: "$0.47/day",
+    highlight: false,
+  },
+] as const;