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/components/table-viewer/TableDataView.tsx

@@ -104,7 +104,7 @@ export function TableDataView({ connectionId, schema, table }: Props) {
     }
     setIsSaving(true);
     try {
-      for (const [_key, change] of pendingChanges) {
+      for (const [, change] of pendingChanges) {
         const row = data.rows[change.rowIndex];
         const colName = data.columns[change.colIndex];