UI::Button uses `foreground_color` — consider adding `text_color` alias

[crimson-knight]

Context

When styling a UI::Button, the property to set the button's text color is foreground_color. While this is technically correct (foreground vs background), the more intuitive name for button text styling is text_color, which aligns with UI::Label and UI::Text conventions.

Problem

button = ::UI::Button.new("Click me")
button.text_color = ::UI::Color.new(1.0, 1.0, 1.0, 1.0)       # Expected — doesn't exist
button.foreground_color = ::UI::Color.new(1.0, 1.0, 1.0, 1.0)  # Actual property

Other text-bearing components like UI::Label use text_color. The inconsistency between text_color on labels and foreground_color on buttons causes confusion.

Proposal

Add text_color as an alias for foreground_color on UI::Button, or standardize on one name across all text-bearing components. Consistency is more important than which name wins.

Options:

1. Add text_color alias on Button (delegates to foreground_color)
2. Add foreground_color alias on Label/Text (delegates to text_color)
3. Standardize all to text_color (breaking change on Button)

Option 1 is the least disruptive.