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/common/PageLoader.tsx

@@ -0,0 +1,12 @@
+export function PageLoader() {
+  return (
+    <div className="flex h-screen items-center justify-center bg-background">
+      <div className="flex flex-col items-center gap-4">
+        <div className="h-12 w-12 animate-spin rounded-full border-4 border-primary border-t-transparent" />
+        <p className="text-sm text-muted-foreground">Loading...</p>
+      </div>
+    </div>
+  )
+}
+
+export default PageLoader

frontend/src/components/common/StarRating.tsx

@@ -0,0 +1,64 @@
+import { Star } from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+interface StarRatingProps {
+  value: number
+  onChange?: (value: number) => void
+  readonly?: boolean
+  size?: 'sm' | 'md' | 'lg'
+  showCount?: boolean
+}
+
+const sizeClasses = {
+  sm: 'h-4 w-4',
+  md: 'h-5 w-5',
+  lg: 'h-6 w-6'
+}
+
+export function StarRating({
+  value,
+  onChange,
+  readonly = false,
+  size = 'md',
+  showCount = false
+}: StarRatingProps) {
+  const handleClick = (rating: number) => {
+    if (!readonly && onChange) {
+      onChange(rating)
+    }
+  }
+
+  return (
+    <div className="flex items-center gap-1">
+      {[1, 2, 3, 4, 5].map((star) => (
+        <button
+          key={star}
+          type="button"
+          onClick={() => handleClick(star)}
+          disabled={readonly}
+          className={cn(
+            'transition-colors',
+            !readonly && 'hover:scale-110 cursor-pointer',
+            readonly && 'cursor-default'
+          )}
+          aria-label={`${star} star${star !== 1 ? 's' : ''}`}
+        >
+          <Star
+            className={cn(
+              sizeClasses[size],
+              star <= value
+                ? 'fill-yellow-400 text-yellow-400'
+                : 'fill-none text-muted-foreground',
+              !readonly && 'hover:text-yellow-300'
+            )}
+          />
+        </button>
+      ))}
+      {showCount && (
+        <span className="ml-1 text-sm text-muted-foreground">
+          ({value}/5)
+        </span>
+      )}
+    </div>
+  )
+}