Expand test coverage for TUI components

[jeanmachuca]

What

Only 2 test files exist (392 lines). Missing tests for:

• keybind.go (keybinding handling)
• styles.go (lipgloss styles and themes)
• No Bubble Tea model integration tests

Why

Bubble Tea supports tea.NewProgram testing, which should be used to verify model/view/update behavior.

How

• Add tests for keybind.go covering all key sequences
• Add tests for styles.go verifying style output
• Add at least one integration test using tea.NewProgram with mock socket