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/api/hooks.ts

@@ -669,6 +669,7 @@ export interface UnifiedContact {
   tags?: string | null;
   image?: string | null;
   custom?: Record<string, any> | null;
+  folder_id?: string | null;
   default_person_id?: string | null;
   admin_contactperson_id?: string | null;
   contact_persons?: ContactPerson[];
@@ -683,14 +684,16 @@ export function useUnifiedContacts(
   contactType?: string,
   sortBy?: string,
   sortOrder?: string,
+  folderId?: string,
 ) {
   const params = new URLSearchParams({ page: String(page), page_size: String(pageSize) });
   if (search) params.set('search', search);
   if (contactType) params.set('type', contactType);
   if (sortBy) params.set('sort_by', sortBy);
   if (sortOrder) params.set('sort_order', sortOrder);
+  if (folderId) params.set('folder_id', folderId);
   return useQuery({
-    queryKey: ['unifiedContacts', page, pageSize, search, contactType, sortBy, sortOrder],
+    queryKey: ['unifiedContacts', page, pageSize, search, contactType, sortBy, sortOrder, folderId],
     queryFn: () =>
       apiGet<PaginatedResponse<UnifiedContact>>(`/contacts?${params.toString()}`),
   });
@@ -1240,3 +1243,55 @@ export function useUserGroups(userId: string | null) {
     enabled: !!userId,
   });
 }
+
+// ── Contact Folders ──
+
+export function useContactFolders() {
+  return useQuery({
+    queryKey: ['contactFolders'],
+    queryFn: () => apiGet<import('./contactFolders').ContactFolder[]>('/contact-folders'),
+  });
+}
+
+export function useCreateContactFolder() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (data: { name: string; parent_id?: string }) => apiPost('/contact-folders', data),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['contactFolders'] });
+    },
+  });
+}
+
+export function useUpdateContactFolder() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ id, data }: { id: string; data: { name?: string; parent_id?: string | null; sort_order?: number } }) =>
+      apiPut(`/contact-folders/${id}`, data),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['contactFolders'] });
+    },
+  });
+}
+
+export function useDeleteContactFolder() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (id: string) => apiDelete(`/contact-folders/${id}`),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['contactFolders'] });
+    },
+  });
+}
+
+export function useMoveContactToFolder() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ contactId, folderId }: { contactId: string; folderId: string | null }) =>
+      apiPut(`/contact-folders/contacts/${contactId}/move`, { folder_id: folderId }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['unifiedContacts'] });
+      queryClient.invalidateQueries({ queryKey: ['contactFolders'] });
+    },
+  });
+}