feat: implement My Trees, admin UI, rating modal, and bundle optimization (Issues #15, #18, #19, #31)

Frontend features:
- My Trees personal dashboard with fork tracking (Issue #15)
- Tree sharing UI with token generation and copy (Issue #16)
- Draft tree badges and validation UI (Issue #25)
- Save session as tree modal (Issue #17)
- Rate/review modal with localStorage tracking (Issue #19)
- Admin category management with drag-and-drop (Issue #18)
- Bundle size optimization with code splitting (Issue #31)

Components created:
- MyTreesPage: Personal tree organization
- AdminCategoriesPage: Category CRUD with @dnd-kit
- ShareTreeModal: Tree sharing interface
- SaveSessionAsTreeModal: Session conversion UI
- StepRatingModal: Post-session rating with stars
- StarRating: Reusable rating component
- PageLoader: Loading fallback for lazy routes
- CreateCategoryModal, EditCategoryModal: Admin modals

Bundle optimization:
- Reduced from 892 KB to 221 KB (75% reduction)
- Dynamic imports for 9 heavy pages
- Vendor chunk splitting for optimal caching
- 6 separate vendor chunks (react, markdown, utils, dnd, icons, state)

Dependencies added:
- @dnd-kit/core, @dnd-kit/sortable, @dnd-kit/utilities

API clients:
- stepCategories: Full CRUD for admin
- Enhanced sessions: saveAsTree endpoint
- Enhanced trees: share, fork, canPublish endpoints

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/components/layout/AppLayout.tsx

@@ -12,7 +12,7 @@ export function AppLayout() {
   const location = useLocation()
   const navigate = useNavigate()
   const { user, logout } = useAuthStore()
-  const { effectiveRole } = usePermissions()
+  const { effectiveRole, isSuperAdmin } = usePermissions()
   const [mobileMenuOpen, setMobileMenuOpen] = useState(false)
 
   const handleLogout = async () => {
@@ -48,9 +48,11 @@ export function AppLayout() {
 
   const navItems = [
     { path: '/trees', label: 'Trees' },
+    { path: '/my-trees', label: 'My Trees' },
     { path: '/sessions', label: 'Sessions' },
     { path: '/account', label: 'Account' },
     { path: '/settings', label: 'Settings' },
+    ...(isSuperAdmin ? [{ path: '/admin/categories', label: 'Admin: Categories' }] : []),
   ]
 
   return (