fix(tui): pass fg prop to diff component for light mode support

[code-yeongyu]

Fix unreadable diff text on light mode themes by passing fg prop to diff component.

Problem

When using light mode themes, syntax-unhighlighted tokens in diff view render as white text (hardcoded default in opentui's CodeRenderable), making them invisible on light backgrounds.

Solution

Pass fg={theme.text} to the <diff> component so unhighlighted text uses the theme's text color.

Dependency

Requires sst/opentui#390 to be merged first (adds fg prop support to DiffRenderable).