feat: add hierarchical folder browser for library creation

- Extend API /folders endpoint to support browsing subdirectories with path parameter
- Add depth and has_children fields to FolderItem
- Create FolderBrowser component with tree view navigation
- Create FolderPicker component with input, browse button and popup modal
- Add API proxy route for /api/folders
- Update LibraryForm to use new FolderPicker component
- Fix path handling to correctly resolve /libraries/ subdirectories

apps/backoffice/lib/api.ts

@@ -41,6 +41,8 @@ export type TokenDto = {
 export type FolderItem = {
   name: string;
   path: string;
+  depth: number;
+  has_children: boolean;
 };
 
 export type BookDto = {
@@ -177,8 +179,9 @@ export async function cancelJob(id: string) {
   return apiFetch<IndexJobDto>(`/index/cancel/${id}`, { method: "POST" });
 }
 
-export async function listFolders() {
-  return apiFetch<FolderItem[]>("/folders");
+export async function listFolders(path?: string) {
+  const url = path ? `/folders?path=${encodeURIComponent(path)}` : "/folders";
+  return apiFetch<FolderItem[]>(url);
 }
 
 export async function listTokens() {