Add focus management system with customizable key bindings

[meszmate]

• Add FocusGroup component for Tab/Shift+Tab cycling between focusable components
• Add FocusStyle helper for focus-aware border rendering
• Add KeyBind struct with fully customizable navigation keys (arrows, vim, modifiers)
• Add isFocusable comptime protocol check
• Add focusable protocol to List, Confirm, and FilePicker components
• Add focus_form example demonstrating Tab cycling with 3 text inputs
• Add 26+ unit tests covering cycling, wrapping, custom keys, and modifier matching
• Update README with focus management docs and custom key binding examples