fix(ui5-calendar): add visible text content to week number column header

[hinzzx]

Overview

Currently in our component ui5-calendar the week number column header uses aria-label instead of visible text content, violating WCAG 2.2 and ARIA 1.2 requirements.

Problems:

1. Empty column header for week numbers: The week number column header rendered as:

<div role="columnheader" aria-label="Calendar Week" class="ui5-dp-dayname">
    <!-- EMPTY! No visible text content -->
</div>

2. Violates WCAG 2.2 Success Criterion 1.3.1 (Info and Relationships):
   • Table headers (including grid column headers) should have actual text content
   • Using only aria-label without visible text is a pattern violation
   • Screen readers rely on actual DOM content, not just ARIA attributes
3. Violates ARIA 1.2 Best Practices:
   • aria-label should supplement, not replace, visible text
   • Column headers in grids/tables require real text content for proper semantic structure
4. Different treatment for different headers:
   • Day name headers: Had visible abbreviated text (M, T, W) + aria-label (Monday, Tuesday, Wednesday)
   • Week number header: Had NO visible text, only aria-label

Now

• Follows WCAG 2.2 1.3.1: Column headers now have actual text content in the DOM
• Follows ARIA 1.2 Table Patterns: Grid column headers contain real text, not just ARIA attributes
• Maintains Visual Design: Week number column still appears as a narrow column with no visible label
• Improves Screen Reader Experience: Text is read from DOM content, which is more reliable than aria-label
• Maintains Abbreviated Day Names: Day columns still show "M, T, W..." with full names in aria-label

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-12975--ui5-webcomponents-preview.netlify.app