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/router.tsx

@@ -1,18 +1,24 @@
 import { createBrowserRouter, Navigate } from 'react-router-dom'
+import { lazy, Suspense } from 'react'
 import { AppLayout, ProtectedRoute } from '@/components/layout'
 import { RouteError } from '@/components/common/RouteError'
+import { PageLoader } from '@/components/common/PageLoader'
 import {
   LoginPage,
   RegisterPage,
-  TreeLibraryPage,
-  TreeNavigationPage,
-  TreeEditorPage,
-  SessionHistoryPage,
-  SessionDetailPage,
-  SettingsPage,
-  AccountSettingsPage,
 } from '@/pages'
 
+// Lazy load heavy pages for code splitting
+const TreeLibraryPage = lazy(() => import('@/pages/TreeLibraryPage'))
+const MyTreesPage = lazy(() => import('@/pages/MyTreesPage'))
+const TreeNavigationPage = lazy(() => import('@/pages/TreeNavigationPage'))
+const TreeEditorPage = lazy(() => import('@/pages/TreeEditorPage'))
+const SessionHistoryPage = lazy(() => import('@/pages/SessionHistoryPage'))
+const SessionDetailPage = lazy(() => import('@/pages/SessionDetailPage'))
+const SettingsPage = lazy(() => import('@/pages/SettingsPage'))
+const AccountSettingsPage = lazy(() => import('@/pages/AccountSettingsPage'))
+const AdminCategoriesPage = lazy(() => import('@/pages/AdminCategoriesPage'))
+
 export const router = createBrowserRouter([
   {
     path: '/login',
@@ -39,35 +45,85 @@ export const router = createBrowserRouter([
       },
       {
         path: 'trees',
-        element: <TreeLibraryPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <TreeLibraryPage />
+          </Suspense>
+        ),
+      },
+      {
+        path: 'my-trees',
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <MyTreesPage />
+          </Suspense>
+        ),
       },
       {
         path: 'trees/new',
-        element: <TreeEditorPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <TreeEditorPage />
+          </Suspense>
+        ),
       },
       {
         path: 'trees/:id/edit',
-        element: <TreeEditorPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <TreeEditorPage />
+          </Suspense>
+        ),
       },
       {
         path: 'trees/:id/navigate',
-        element: <TreeNavigationPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <TreeNavigationPage />
+          </Suspense>
+        ),
       },
       {
         path: 'sessions',
-        element: <SessionHistoryPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <SessionHistoryPage />
+          </Suspense>
+        ),
       },
       {
         path: 'sessions/:id',
-        element: <SessionDetailPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <SessionDetailPage />
+          </Suspense>
+        ),
       },
       {
         path: 'settings',
-        element: <SettingsPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <SettingsPage />
+          </Suspense>
+        ),
       },
       {
         path: 'account',
-        element: <AccountSettingsPage />,
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <AccountSettingsPage />
+          </Suspense>
+        ),
+      },
+      {
+        path: 'admin/categories',
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <ProtectedRoute requiredRole="super_admin">
+              <AdminCategoriesPage />
+            </ProtectedRoute>
+          </Suspense>
+        ),
       },
     ],
   },