Removes t.Parallel() from TestRadioMenu_Unit, TestShowVersion_Unit,
and TestPrecheck_Unit. These tests mutate globals, redirect os.Stdout,
or call external dependencies, causing races when run concurrently
with other tests under -race.
Added comprehensive unit and live integration tests in new files:
- commander_test.go: Covers isInstalled and subExecute functions
- filer_test.go: Covers createIniFile with various scenarios
- radiomenu_test.go: Covers Short, Quit, and RadioMenu
- stations_test.go: Covers showVersion and precheck
These tests ensure reliability across happy paths, edge cases, and live environments.
