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/sidebar/components/sidebar-screen.tsx

@@ -76,6 +76,11 @@ export function SidebarScreen() {
           <SettingsSection
             title="Other"
             items={[
+              {
+                id: "membership",
+                title: "Membership",
+                onClick: () => router.push(ROUTES.subscription),
+              },
               {
                 id: "logout",
                 title: "Log out",