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

Cargo.lock

@@ -8,22 +8,6 @@ version = "2.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "320119579fcad9c21884f5c4861d16174d0e06250625266f50fe6898340abefa"
 
-[[package]]
-name = "admin-ui"
-version = "0.1.0"
-dependencies = [
- "anyhow",
- "axum",
- "reqwest",
- "serde",
- "serde_json",
- "stripstream-core",
- "tokio",
- "tracing",
- "tracing-subscriber",
- "uuid",
-]
-
 [[package]]
 name = "aes"
 version = "0.8.4"