fix: resolve clippy warnings and test errors

- Implement FromStr trait for NodeType instead of custom from_str method
- Fix redundant_closure warning in server.rs:455
- Add #[allow(clippy::too_many_arguments)] for new_file_node
- Fix unused variables in tests (_user_id, _conn)
- Remove unused imports (NodeType, serde_json::json)
- Replace len() > 0 with !is_empty() for clarity
- Replace == false with negation operator
- Format code with cargo fmt

tests/filetree_api_test.rs

@@ -1,6 +1,5 @@
 use markbase::filetree::{node::NodeType, FileTree};
 use rusqlite::Connection;
-use serde_json::json;
 use uuid::Uuid;
 
 fn temp_db() -> (Connection, String) {
@@ -207,7 +206,7 @@ fn test_api_build_tree_structure() {
 
 #[test]
 fn test_api_add_location() {
-    let (conn, user_id) = temp_db();
+    let (_conn, user_id) = temp_db();
     let conn = FileTree::open_user_db(&user_id).unwrap();
 
     let file_uuid = "abc123def456";