Clarify default GUI usage

[galjos]

Summary

• show GUI mode as optional in top-level usage
• include format flags and file arguments in the usage line
• cover the generated help text

Tests

• 204 tests with 100.00% configured coverage
• 9 GUI/TUI end-to-end tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2699e5f) to head (6caab20).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #92   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          502       502           
=========================================
  Hits           502       502           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.