Fix selected segment title color for Light Mode

[umairzahid508]

Summary

In Light Mode, standardContent.inverted resolved to white, causing the selected segment title to render white-on-white when selectedSegmentTintColor is also white. This made the selected segment title invisible in Light Mode.

Fix

• Added static var selectedContent : EKColor{ return EKColor(light: .black, dark: .black) } to EKColor.
• Updated EKSegmentedControl to use EKColor.selectedContent.color(for:mode:) for .selected state instead of standardContent.inverted.

Rationale

Using a dedicated selectedContent color guarantees sufficient contrast for selected segment titles in both Light and Dark mode while preserving existing theme architecture.

Tests / Verification

• Build succeeds locally.
• Confirmed visually that selected segment titles are readable in Light and Dark modes.
• Ran unit tests.

Notes for reviewer

• Change is minimal and localized to EKColor and EKSegmentedControl.
• No breaking API changes.