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

architectural-grid_landing/.env.example

@@ -0,0 +1,9 @@
+# GEMINI_API_KEY: Required for Gemini AI API calls.
+# AI Studio automatically injects this at runtime from user secrets.
+# Users configure this via the Secrets panel in the AI Studio UI.
+GEMINI_API_KEY="MY_GEMINI_API_KEY"
+
+# APP_URL: The URL where this applet is hosted.
+# AI Studio automatically injects this at runtime with the Cloud Run service URL.
+# Used for self-referential links, OAuth callbacks, and API endpoints.
+APP_URL="MY_APP_URL"