Add accessible name to header navigation blocks

[shail-mehta]

Description

Fixes #100.

Adds a translatable ariaLabel of Primary to the Navigation block in the header patterns (header-default, header-stick, header-collapsed).

Without an ariaLabel, the block falls back to aria-label="Navigation". Accessibility-ready landmark guidance asks for a name that does not include the landmark type (e.g. Primary, not Primary navigation).

Screenshots
N/A - change is an aria-label on the header <nav>.

Testing Instructions

1. Activate Ipsum and open the front page with the default header.
2. Inspect the header <nav> - it should have aria-label="Primary" (not Navigation).
3. In the Site Editor, switch the header template part to Header Stick, then Header Collapsed; reload the front end and confirm the same aria-label on each.

Use of AI Tools
Cursor (For PR Description)

[github-actions]

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Co-authored-by: shail-mehta <shailu25@git.wordpress.org>
Co-authored-by: carolinan <poena@git.wordpress.org>

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[carolinan]

Thank you