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/vite.config.ts

@@ -10,4 +10,36 @@ export default defineConfig({
       '@': path.resolve(__dirname, './src'),
     },
   },
+  build: {
+    rollupOptions: {
+      output: {
+        manualChunks: {
+          // React core and routing
+          'react-vendor': ['react', 'react-dom', 'react-router-dom'],
+          // Markdown rendering
+          'markdown-vendor': ['react-markdown'],
+          // State management
+          'state-vendor': ['zustand', 'immer', 'zundo'],
+          // Icons
+          'icons-vendor': ['lucide-react'],
+          // Utilities and UI libs
+          'utils-vendor': [
+            'axios',
+            'clsx',
+            'tailwind-merge',
+            'class-variance-authority',
+            'date-fns',
+            'sonner',
+          ],
+          // Drag and drop
+          'dnd-vendor': [
+            '@dnd-kit/core',
+            '@dnd-kit/sortable',
+            '@dnd-kit/utilities',
+          ],
+        },
+      },
+    },
+    chunkSizeWarningLimit: 500,
+  },
 })