feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

memento-note/lib/billing/stripe-prices.ts

@@ -0,0 +1,35 @@
+import type { SubscriptionTier } from '@/lib/entitlements';
+
+export type BillingTier = 'PRO' | 'BUSINESS';
+export type BillingInterval = 'month' | 'year';
+
+export function resolvePriceId(tier: BillingTier, interval: BillingInterval): string {
+  const map: Record<BillingTier, Record<BillingInterval, string>> = {
+    PRO: {
+      month: process.env.STRIPE_PRICE_PRO_MONTHLY!,
+      year: process.env.STRIPE_PRICE_PRO_ANNUAL!,
+    },
+    BUSINESS: {
+      month: process.env.STRIPE_PRICE_BUSINESS_MONTHLY!,
+      year: process.env.STRIPE_PRICE_BUSINESS_ANNUAL!,
+    },
+  };
+  const priceId = map[tier][interval];
+  if (!priceId) {
+    throw new Error(`No Stripe price ID configured for ${tier}/${interval}`);
+  }
+  return priceId;
+}
+
+export function priceIdToTier(priceId: string): SubscriptionTier | null {
+  const entries: Array<[string | undefined, SubscriptionTier]> = [
+    [process.env.STRIPE_PRICE_PRO_MONTHLY, 'PRO'],
+    [process.env.STRIPE_PRICE_PRO_ANNUAL, 'PRO'],
+    [process.env.STRIPE_PRICE_BUSINESS_MONTHLY, 'BUSINESS'],
+    [process.env.STRIPE_PRICE_BUSINESS_ANNUAL, 'BUSINESS'],
+  ];
+  for (const [envPriceId, tier] of entries) {
+    if (envPriceId && envPriceId === priceId) return tier;
+  }
+  return null;
+}