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/api/sessions.ts

@@ -1,5 +1,5 @@
 import apiClient from './client'
-import type { Session, SessionCreate, SessionUpdate, SessionExport } from '@/types'
+import type { Session, SessionCreate, SessionUpdate, SessionExport, SaveAsTreeRequest, SaveAsTreeResponse } from '@/types'
 
 export interface SessionListParams {
   page?: number
@@ -58,6 +58,11 @@ export const sessionsApi = {
     const response = await apiClient.patch<Session>(`/sessions/${id}/scratchpad`, { scratchpad: content })
     return response.data
   },
+
+  async saveAsTree(id: string, data: SaveAsTreeRequest): Promise<SaveAsTreeResponse> {
+    const response = await apiClient.post<SaveAsTreeResponse>(`/sessions/${id}/save-as-tree`, data)
+    return response.data
+  },
 }
 
 export default sessionsApi