Add folder permissions UI and WebDAV protocol support

Permissions UI:
- FolderPermissionsDialog component with public/private toggle,
  role/user permission management, and access level badges
- Integrated into file manager toolbar (visible for folder admins)
- Backend returns accessLevel in folder detail endpoint

WebDAV server:
- Full WebDAV protocol at /webdav/ with Basic Auth (existing credentials)
- PROPFIND, GET, PUT, DELETE, MKCOL, COPY, MOVE, LOCK/UNLOCK support
- Permission-checked against existing folder access model
- In-memory lock stubs for Windows client compatibility
- 22 API integration tests covering all operations

Also fixes canAccess to check folder creator (was missing).

packages/admin/src/routes/_authenticated/files/index.tsx

@@ -17,9 +17,10 @@ import { Label } from '@/components/ui/label'
 import {
   DropdownMenu, DropdownMenuContent, DropdownMenuItem, DropdownMenuTrigger,
 } from '@/components/ui/dropdown-menu'
-import { FolderPlus, Upload, Search, Folder, ChevronRight, MoreVertical, Trash2, Download } from 'lucide-react'
+import { FolderPlus, Upload, Search, Folder, ChevronRight, MoreVertical, Trash2, Download, Shield } from 'lucide-react'
 import { toast } from 'sonner'
 import { useAuthStore } from '@/stores/auth.store'
+import { FolderPermissionsDialog } from '@/components/storage/folder-permissions-dialog'
 import type { StorageFolder, StorageFile } from '@/types/storage'
 
 export const Route = createFileRoute('/_authenticated/files/')({
@@ -39,6 +40,7 @@ function FileManagerPage() {
   const [selectedFolderId, setSelectedFolderId] = useState<string | null>(null)
   const [newFolderOpen, setNewFolderOpen] = useState(false)
   const [newFolderName, setNewFolderName] = useState('')
+  const [permissionsOpen, setPermissionsOpen] = useState(false)
   const fileInputRef = useRef<HTMLInputElement>(null)
   const { params } = usePagination()
 
@@ -193,6 +195,11 @@ function FileManagerPage() {
 
           {selectedFolderId && (
             <>
+              {folderDetail?.accessLevel === 'admin' && (
+                <Button variant="outline" size="sm" onClick={() => setPermissionsOpen(true)}>
+                  <Shield className="mr-2 h-4 w-4" />Permissions
+                </Button>
+              )}
               <Button variant="outline" size="sm" onClick={() => fileInputRef.current?.click()}>
                 <Upload className="mr-2 h-4 w-4" />Upload
               </Button>
@@ -283,6 +290,16 @@ function FileManagerPage() {
           )}
         </div>
       </div>
+
+      {selectedFolderId && folderDetail && (
+        <FolderPermissionsDialog
+          folderId={selectedFolderId}
+          folderName={folderDetail.name ?? ''}
+          isPublic={folderDetail.isPublic ?? true}
+          open={permissionsOpen}
+          onOpenChange={setPermissionsOpen}
+        />
+      )}
     </div>
   )
 }