feat: Kontakt-Ordner mit Drag&Drop Baum (wie KI-Chat Sidebar)

- Backend: ContactFolder model (hierarchisch, parent_id, sort_order)
- Migration 0022: contact_folders Tabelle + folder_id FK auf contacts
- CRUD Routes: /api/v1/contact-folders (list, create, update, delete, reorder)
- Move Contact API: /api/v1/contact-folders/contacts/{id}/move
- folder_id Filter in contacts list API
- Frontend: ContactFolderTree mit Baum-Struktur, Drag&Drop, Kontext-Menü
- Kontakt-Personen-Icon statt Ordner-Symbol
- ContactList Items draggable (List/Table/Cards View)
- React Query Hooks für Folder CRUD + Move Contact
- Alle 266 Frontend-Tests passing

frontend/src/__tests__/contacts/ContactsList.test.tsx

@@ -59,6 +59,11 @@ vi.mock('@/api/hooks', () => ({
   useCreateContactPerson: () => ({ mutateAsync: vi.fn(), isPending: false }),
   useUpdateContactPerson: () => ({ mutateAsync: vi.fn(), isPending: false }),
   useDeleteContactPerson: () => ({ mutateAsync: vi.fn(), isPending: false }),
+  useContactFolders: () => ({ data: [], isLoading: false }),
+  useCreateContactFolder: () => ({ mutate: vi.fn(), isPending: false }),
+  useUpdateContactFolder: () => ({ mutate: vi.fn(), isPending: false }),
+  useDeleteContactFolder: () => ({ mutate: vi.fn(), isPending: false }),
+  useMoveContactToFolder: () => ({ mutate: vi.fn(), isPending: false }),
 }));
 
 vi.mock('@/components/ui/Toast', () => ({