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/pages/index.ts

@@ -1,9 +1,11 @@
 export { default as LoginPage } from './LoginPage'
 export { default as RegisterPage } from './RegisterPage'
 export { default as TreeLibraryPage } from './TreeLibraryPage'
+export { default as MyTreesPage } from './MyTreesPage'
 export { default as TreeNavigationPage } from './TreeNavigationPage'
 export { default as TreeEditorPage } from './TreeEditorPage'
 export { default as SessionHistoryPage } from './SessionHistoryPage'
 export { default as SessionDetailPage } from './SessionDetailPage'
 export { default as SettingsPage } from './SettingsPage'
 export { default as AccountSettingsPage } from './AccountSettingsPage'
+export { default as AdminCategoriesPage } from './AdminCategoriesPage'