[FIX] Keep lightness active menu items readable.

[middleDuckAi]

Closes #2287

What changed

• keep active and hovered navbar items readable in the default lightness theme
• remove the white active-state override for the lightness menu so selected items use the same dark text as the rest of the theme
• replace the earlier too-narrow menu-toggle regression with a CSS contract test for the real lightness case

Why

The issue is not limited to the theme switcher. On the default lightness theme, active menu items in the main navbar were forced to white text, which made them nearly invisible against the light background.

Validation

• php -l core/tests/Unit/Manager/LightnessActiveMenuContrastTest.php
• direct CSS contract check for the lightness active/hover selectors