Add tree organization system with categories, tags, and folders

Features:
- Categories: Global and team-specific tree categorization (admin-managed)
- Tags: Flexible tree tagging with autocomplete (author + admin)
- User folders: Personal tree collections with subfolder support
  - Hierarchical structure (max 3 levels deep)
  - Right-click context menu for folder management
  - Cascade delete for subfolders
- Filter trees by category, tags, and folder in library view

Backend:
- New models: Category, Tag, UserFolder with relationships
- New API endpoints for categories, tags, and folders
- Tree organization migrations (005, 006)

Frontend:
- FolderSidebar with hierarchical folder tree
- FolderEditModal for create/edit with color picker
- AddToFolderMenu for quick tree organization
- TagInput with autocomplete and TagBadges display
- Updated TreeMetadataForm and TreeLibraryPage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/TreeEditorPage.tsx

@@ -138,12 +138,14 @@ export function TreeEditorPage() {
       const treeData = getTreeForSave()
       if (isEditMode) {
         await treesApi.update(id!, treeData as TreeUpdate)
+        markSaved()
       } else {
         const newTree = await treesApi.create(treeData as TreeCreate)
+        // Mark saved BEFORE navigating to avoid triggering the blocker
+        markSaved()
         // Navigate to edit mode with the new ID
         navigate(`/trees/${newTree.id}/edit`, { replace: true })
       }
-      markSaved()
     } catch (err) {
       console.error('Failed to save tree:', err)
       setSaveError('Failed to save tree. Please try again.')