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-screen.module.css

@@ -0,0 +1,65 @@
+.shell {
+  display: flex;
+  flex-direction: column;
+  min-height: 100dvh;
+  background: var(--color-page-background);
+  padding: 0 var(--spacing-lg) var(--spacing-xxl);
+}
+
+.header {
+  display: flex;
+  align-items: center;
+  height: 40px;
+  padding-top: var(--spacing-sm);
+}
+
+.backSlot {
+  display: inline-flex;
+}
+
+.userSlot {
+  margin-top: var(--spacing-md);
+  padding: 0 var(--spacing-xs);
+}
+
+.bannerSlot {
+  margin-top: var(--spacing-lg);
+}
+
+.plansRow {
+  display: grid;
+  grid-template-columns: repeat(3, 1fr);
+  gap: var(--spacing-sm);
+  margin-top: var(--spacing-xl);
+}
+
+.autoRenewCaption {
+  margin: var(--spacing-md) 0 0;
+  padding: 0 var(--spacing-xs);
+  font-size: var(--font-size-sm);
+  color: var(--color-text-secondary);
+  line-height: 1.5;
+}
+
+.benefitsSlot {
+  margin-top: var(--spacing-lg);
+}
+
+.ctaSlot {
+  margin-top: var(--spacing-xl);
+}
+
+.agreementSlot {
+  margin-top: var(--spacing-md);
+  padding: 0 var(--spacing-xs);
+}
+
+.agreementLink {
+  color: var(--color-auth-text-primary);
+  font-weight: 700;
+  text-decoration: none;
+}
+
+.agreementLink:hover {
+  text-decoration: underline;
+}