Add visibility toggle helper and integration test

[multiplex55]

Summary

• share a mock egui::Context for tests
• factor out visibility toggle logic into src/visibility.rs
• expose the helper via multi_launcher::visibility
• reuse mock context in existing gui_visibility test
• add new integration test exercising visibility toggle when context is present

Testing

• cargo test --quiet (fails: The system library xi required by crate x11 was not found)