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/library/TreeGridView.tsx

@@ -1,5 +1,5 @@
 import { Link } from 'react-router-dom'
-import { Pencil, Globe, Lock, Trash2 } from 'lucide-react'
+import { Pencil, Globe, Lock, Trash2, GitBranch, FileText } from 'lucide-react'
 import type { TreeListItem } from '@/types'
 import { TagBadges } from '@/components/common/TagBadges'
 import { AddToFolderMenu } from './AddToFolderMenu'
@@ -12,6 +12,7 @@ interface TreeGridViewProps {
   onTagClick: (tag: string) => void
   onFolderCreated: (parentId?: string | null) => void
   onDeleteTree: (tree: TreeListItem) => void
+  onForkTree?: (treeId: string) => void
 }
 
 export function TreeGridView({
@@ -20,6 +21,7 @@ export function TreeGridView({
   onTagClick,
   onFolderCreated,
   onDeleteTree,
+  onForkTree,
 }: TreeGridViewProps) {
   const { canEditTree, canDeleteTree } = usePermissions()
 
@@ -31,7 +33,15 @@ export function TreeGridView({
           className="rounded-lg border border-border bg-card p-4 shadow-sm transition-all hover:-translate-y-0.5 hover:border-primary/30 hover:shadow-md sm:p-6"
         >
           <div className="mb-2 flex items-start justify-between gap-2">
-            <h3 className="font-semibold text-card-foreground">{tree.name}</h3>
+            <div className="flex items-center gap-2">
+              <h3 className="font-semibold text-card-foreground">{tree.name}</h3>
+              {tree.status === 'draft' && (
+                <span className="inline-flex items-center gap-1 rounded-full bg-yellow-100 px-2 py-0.5 text-xs font-medium text-yellow-800 dark:bg-yellow-900/30 dark:text-yellow-400">
+                  <FileText className="h-3 w-3" />
+                  Draft
+                </span>
+              )}
+            </div>
             <div className="flex items-center gap-2">
               {tree.is_public ? (
                 <span title="Public tree">
@@ -66,6 +76,19 @@ export function TreeGridView({
             </span>
             <div className="flex items-center gap-2">
               <AddToFolderMenu treeId={tree.id} onFolderCreated={onFolderCreated} />
+              {onForkTree && (
+                <button
+                  type="button"
+                  onClick={() => onForkTree(tree.id)}
+                  className={cn(
+                    'rounded-md border border-input p-2 text-muted-foreground',
+                    'hover:bg-accent hover:text-accent-foreground'
+                  )}
+                  title="Fork tree"
+                >
+                  <GitBranch className="h-4 w-4" />
+                </button>
+              )}
               {canEditTree({ author_id: tree.author_id, account_id: tree.account_id }) && (
                 <Link
                   to={`/trees/${tree.id}/edit`}