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/router/routes.ts

@@ -21,6 +21,7 @@ export const ROUTES = {
   chat: "/chat",
   auth: "/auth",
   sidebar: "/sidebar",
+  subscription: "/subscription",
 } as const;
 
 export type StaticRoute = (typeof ROUTES)[keyof typeof ROUTES];
@@ -41,6 +42,7 @@ export const AUTH_ONLY_ROUTES: readonly StaticRoute[] = [
 export const PROTECTED_ROUTES: readonly StaticRoute[] = [
   ROUTES.chat,
   ROUTES.sidebar,
+  ROUTES.subscription,
 ] as const;
 
 /** 公开路由（游客态也允许访问） */