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/app/components/LibraryForm.tsx

@@ -0,0 +1,38 @@
+"use client";
+
+import { useState } from "react";
+import { FolderPicker } from "./FolderPicker";
+import { FolderItem } from "../../lib/api";
+import { Button, FormField, FormInput, FormRow } from "./ui";
+
+interface LibraryFormProps {
+  initialFolders: FolderItem[];
+  action: (formData: FormData) => void;
+}
+
+export function LibraryForm({ initialFolders, action }: LibraryFormProps) {
+  const [selectedPath, setSelectedPath] = useState<string>("");
+
+  return (
+    <form action={action}>
+      <FormRow>
+        <FormField className="flex-1 min-w-48">
+          <FormInput name="name" placeholder="Library name" required />
+        </FormField>
+        <FormField className="flex-1 min-w-64">
+          <input type="hidden" name="root_path" value={selectedPath} />
+          <FolderPicker
+            initialFolders={initialFolders}
+            selectedPath={selectedPath}
+            onSelect={setSelectedPath}
+          />
+        </FormField>
+      </FormRow>
+      <div className="mt-4 flex justify-end">
+        <Button type="submit" disabled={!selectedPath}>
+          Add Library
+        </Button>
+      </div>
+    </form>
+  );
+}