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/components/contacts/ContactList.tsx

@@ -113,6 +113,11 @@ export function ContactList({
             {contacts.map((contact) => (
               <li key={contact.id}>
                 <button
+                  draggable
+                  onDragStart={(e) => {
+                    e.dataTransfer.setData('text/plain', contact.id);
+                    e.dataTransfer.effectAllowed = 'move';
+                  }}
                   onClick={() => onSelectContact(contact)}
                   className={clsx(
                     'flex items-center gap-3 w-full px-3 py-2.5 text-left min-h-touch transition-colors',
@@ -201,6 +206,11 @@ export function ContactList({
               {contacts.map((contact) => (
                 <tr
                   key={contact.id}
+                  draggable
+                  onDragStart={(e) => {
+                    e.dataTransfer.setData('text/plain', contact.id);
+                    e.dataTransfer.effectAllowed = 'move';
+                  }}
                   onClick={() => onSelectContact(contact)}
                   className={clsx(
                     'cursor-pointer transition-colors',
@@ -247,6 +257,11 @@ export function ContactList({
           {contacts.map((contact) => (
             <div
               key={contact.id}
+              draggable
+              onDragStart={(e) => {
+                e.dataTransfer.setData('text/plain', contact.id);
+                e.dataTransfer.effectAllowed = 'move';
+              }}
               onClick={() => onSelectContact(contact)}
               className={clsx(
                 'p-3 rounded-lg border cursor-pointer transition-colors min-h-touch',