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/config.ts

@@ -9,6 +9,12 @@ const ENV_FALLBACKS: Record<string, string> = {
   AI_MODEL_EMBEDDING: process.env.AI_MODEL_EMBEDDING || '',
   AI_PROVIDER_CHAT: process.env.AI_PROVIDER_CHAT || '',
   AI_MODEL_CHAT: process.env.AI_MODEL_CHAT || '',
+  AI_PROVIDER_CHAT_FALLBACK: process.env.AI_PROVIDER_CHAT_FALLBACK || '',
+  AI_MODEL_CHAT_FALLBACK: process.env.AI_MODEL_CHAT_FALLBACK || '',
+  AI_PROVIDER_TAGS_FALLBACK: process.env.AI_PROVIDER_TAGS_FALLBACK || '',
+  AI_MODEL_TAGS_FALLBACK: process.env.AI_MODEL_TAGS_FALLBACK || '',
+  AI_PROVIDER_EMBEDDING_FALLBACK: process.env.AI_PROVIDER_EMBEDDING_FALLBACK || '',
+  AI_MODEL_EMBEDDING_FALLBACK: process.env.AI_MODEL_EMBEDDING_FALLBACK || '',
   // Ollama
   OLLAMA_BASE_URL: process.env.OLLAMA_BASE_URL || '',
   // OpenAI