Improve forced-colors mode accessibility for focus indicators

[coderabbitai]

Description

The EnhancedTabs component does not render reliably in forced-colors (high-contrast) mode. The current focus ring implementation relies on box-shadow which may not be visible in forced-colors contexts, potentially causing keyboard focus to be invisible to users who rely on high-contrast settings.

Context

This issue was identified during review of PR #28, which addresses focus-ring styles for Vaadin 24/25.

Related PR: #28
Related comment: #28 (comment)

Expected Behavior

Focus indicators should remain visible and accessible in forced-colors/high-contrast mode to ensure keyboard navigation remains usable for all users.

Current Behavior

The component has rendering issues in forced-colors mode, including focus indicators that may not be visible.

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Reported by: @javier-godoy