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-back-link.module.css

@@ -0,0 +1,20 @@
+.backLink {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 8px;
+  margin-left: -8px;
+  color: #000;
+  text-decoration: none;
+  border-radius: var(--radius-md);
+}
+
+.backLink:focus-visible {
+  outline: 2px solid var(--color-accent);
+  outline-offset: 2px;
+}
+
+.icon {
+  display: block;
+  color: inherit;
+}