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

docs/sprint-status.yaml

@@ -1,5 +1,5 @@
 # generated: 2026-05-14T16:06:50Z
-# last_updated: 2026-05-14T16:06:50Z
+# last_updated: 2026-05-15T20:00:00Z
 # project: Momento
 # project_key: NOKEY
 # tracking_system: file-system
@@ -35,7 +35,7 @@
 # - Dev moves story to 'review', then runs code-review (fresh context, different LLM recommended)
 
 generated: 2026-05-14T16:06:50Z
-last_updated: 2026-05-14T18:30:00Z
+last_updated: 2026-05-15T20:00:00Z
 project: Momento
 project_key: NOKEY
 tracking_system: file-system
@@ -43,12 +43,12 @@ story_location: docs
 
 development_status:
   epic-3: in-progress
-  3-1-freemium-quota-tracking: ready-for-dev
-  3-2-custom-llm-router: backlog
-  3-3-smart-routing-fallback: backlog
-  3-4-host-pays-session-logic: backlog
-  3-5-secure-byok-management: backlog
-  3-6-stripe-subscription-tiers: backlog
+  3-1-freemium-quota-tracking: done
+  3-2-custom-llm-router: done
+  3-3-smart-routing-fallback: done
+  3-4-host-pays-session-logic: done
+  3-5-secure-byok-management: review
+  3-6-stripe-subscription-tiers: ready-for-dev
   epic-3-retrospective: optional
   epic-4: backlog
   4-1-gdpr-cookie-consent: backlog