feat: contact folder drag-and-drop, mobile dropdown, folder-in-folder move

- Folders are draggable: drag folder into another folder (parent_id update)
- Circular reference prevention: isDescendantOrSelf() check
- Root drop zone: drag folder to root unparents it (parent_id=null)
- MoreVertical button: opacity-60 for mobile visibility (was opacity-0)
- Dropdown: viewport-clamped positioning + maxHeight with scroll
- ContactList already had draggable contacts (no changes needed)

frontend/src/components/contacts/ContactFolderTree.tsx

@@ -8,11 +8,25 @@ import {
   useDeleteContactFolder,
   useMoveContactToFolder,
 } from '@/api/hooks';
-import { buildFolderTree, type ContactFolderTreeNode } from '@/api/contactFolders';
+import { buildFolderTree, type ContactFolderTreeNode, type ContactFolder } from '@/api/contactFolders';
 import { ChevronRight, Folder, MoreVertical, Palette, Pencil, Pin, Plus, Tag, Trash2, Users } from 'lucide-react';
 
 export type ContactFilter = 'all' | 'company' | 'person' | `tag:${string}` | `folder:${string}`;
 
+/**
+ * Returns true if `descendantId` is a descendant of (or equal to) `ancestorId`
+ * in the given flat folder list. Used to prevent circular folder moves.
+ */
+function isDescendantOrSelf(folders: ContactFolder[], descendantId: string, ancestorId: string): boolean {
+  if (descendantId === ancestorId) return true;
+  let current = folders.find((f) => f.id === descendantId);
+  while (current && current.parent_id) {
+    if (current.parent_id === ancestorId) return true;
+    current = folders.find((f) => f.id === current!.parent_id!);
+  }
+  return false;
+}
+
 export interface ContactFolderTreeProps {
   selectedFilter: ContactFilter;
   onSelect: (filter: ContactFilter) => void;
@@ -83,13 +97,14 @@ function FolderDropdown({
   ];
 
   const top = state.anchorRect.bottom + 4;
-  const left = Math.max(4, state.anchorRect.right - 160);
+  const left = Math.max(4, Math.min(state.anchorRect.right - 160, window.innerWidth - 170));
+  const maxHeight = Math.min(300, window.innerHeight - top - 8);
 
   return (
     <div
       ref={ref}
       className="fixed z-50 bg-white border border-secondary-200 rounded-lg shadow-lg py-1 min-w-[160px]"
-      style={{ top, left }}
+      style={{ top, left, maxHeight: maxHeight > 0 ? maxHeight : undefined, overflowY: 'auto' }}
     >
       {items.map((item, i) => (
         <button
@@ -138,6 +153,11 @@ function FolderTreeItem({
   return (
     <div>
       <div
+        draggable
+        onDragStart={(e) => {
+          e.dataTransfer.setData('text/folder-id', node.id);
+          e.dataTransfer.effectAllowed = 'move';
+        }}
         onDragOver={(e) => onDragOver(e, node.id)}
         onDragLeave={() => onDragLeave(node.id)}
         onDrop={(e) => onDrop(e, node.id)}
@@ -164,7 +184,7 @@ function FolderTreeItem({
         )}
         <button
           onClick={(e) => { e.stopPropagation(); onMoreClick(e, node.id); }}
-          className="opacity-0 group-hover:opacity-100 text-secondary-400 hover:text-primary-600 p-0.5"
+          className="opacity-60 group-hover:opacity-100 text-secondary-400 hover:text-primary-600 p-0.5"
           title="Optionen"
           aria-label="Optionen"
         >
@@ -272,6 +292,19 @@ export function ContactFolderTree({
     e.preventDefault();
     e.stopPropagation();
     setDragOverFolderId(null);
+
+    // 1) Folder-to-folder drop: move dragged folder into target folder
+    const draggedFolderId = e.dataTransfer.getData('text/folder-id');
+    if (draggedFolderId) {
+      // Prevent dropping a folder into itself or any of its descendants
+      if (isDescendantOrSelf(folderList, folderId, draggedFolderId)) {
+        return;
+      }
+      updateFolderMut.mutate({ id: draggedFolderId, data: { parent_id: folderId } });
+      return;
+    }
+
+    // 2) Contact-to-folder drop
     const contactId = e.dataTransfer.getData('text/plain');
     if (!contactId) return;
     moveContactMut.mutate({ contactId, folderId });
@@ -279,6 +312,13 @@ export function ContactFolderTree({
 
   const handleRootDrop = (e: React.DragEvent) => {
     e.preventDefault();
+    // Folder-to-root: unparent the dragged folder
+    const draggedFolderId = e.dataTransfer.getData('text/folder-id');
+    if (draggedFolderId) {
+      updateFolderMut.mutate({ id: draggedFolderId, data: { parent_id: null } });
+      return;
+    }
+    // Contact-to-root: remove from any folder
     const contactId = e.dataTransfer.getData('text/plain');
     if (!contactId) return;
     moveContactMut.mutate({ contactId, folderId: null });