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/pages/ContactsList.tsx

@@ -53,6 +53,12 @@ export function ContactsListPage() {
     return undefined;
   }, [selectedFilter]);
 
+  // Derive folder filter
+  const folderId = useMemo(() => {
+    if (selectedFilter.startsWith('folder:')) return selectedFilter.slice(7);
+    return undefined;
+  }, [selectedFilter]);
+
   // Derive tag filter
   const tagFilter = useMemo(() => {
     if (selectedFilter.startsWith('tag:')) return selectedFilter.slice(4);
@@ -67,6 +73,7 @@ export function ContactsListPage() {
     contactType,
     sortBy,
     sortOrder,
+    folderId,
   );
 
   // Fetch selected contact detail (with contact_persons)
@@ -219,6 +226,7 @@ export function ContactsListPage() {
               selectedFilter={selectedFilter}
               onSelect={handleSelectFilter}
               tags={allTags}
+              contacts={contacts}
             />
           </div>
         </ResizablePanel>
@@ -351,6 +359,7 @@ export function ContactsListPage() {
                 selectedFilter={selectedFilter}
                 onSelect={handleSelectFilter}
                 tags={allTags}
+                contacts={contacts}
               />
             </div>
           </div>