style: apply rustfmt, fix clippy warnings, and minor code cleanup

Reformat Rust code with rustfmt, suppress clippy::too_many_arguments
for Tauri IPC commands, derive Default for AppSettings, fix unused
variable pattern in TableDataView, and add unit tests for utils.

src-tauri/src/commands/mod.rs

@@ -9,5 +9,5 @@ pub mod management;
 pub mod queries;
 pub mod saved_queries;
 pub mod schema;
-pub mod snapshot;
 pub mod settings;
+pub mod snapshot;