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/.env.example

@@ -75,3 +75,28 @@ NEXTAUTH_URL="http://localhost:3000"
 # -----------------------------------------------------------------------------
 # MCP_SERVER_MODE="disabled"
 # MCP_SERVER_URL=""
+
+# -----------------------------------------------------------------------------
+# BYOK encryption (Story 3.5 — required in production when users save API keys)
+# -----------------------------------------------------------------------------
+# Generate: openssl rand -base64 32  (minimum 32 characters)
+# MASTER_ENCRYPTION_KEY=""
+
+# -----------------------------------------------------------------------------
+# Stripe (Story 3.6 — subscription tiers)
+# -----------------------------------------------------------------------------
+# Server-side secrets — NEVER expose on client
+# STRIPE_SECRET_KEY="sk_test_..."
+# STRIPE_WEBHOOK_SECRET="whsec_..."
+
+# Price IDs from Stripe Dashboard (create products/prices there first)
+# STRIPE_PRICE_PRO_MONTHLY="price_..."
+# STRIPE_PRICE_PRO_ANNUAL="price_..."
+# STRIPE_PRICE_BUSINESS_MONTHLY="price_..."
+# STRIPE_PRICE_BUSINESS_ANNUAL="price_..."
+
+# Client-side publishable key (safe to expose)
+# NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY="pk_test_..."
+
+# Feature flag — set to "true" to enable billing UI (default: false)
+# NEXT_PUBLIC_FEATURE_BILLING_ENABLED="false"