feat(header): add "Go to header" skip link

[cedric07]

Recommandé par le RGAA

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Note

Low Risk
Low risk: small accessibility markup change (new skip link and tabindex on header) plus routine .pot/French translation updates.

Overview
Improves keyboard accessibility by adding a new skip link (Go to header) in header.php and making the header target focusable via tabindex="-1".

Updates localization files (languages/beapi-frontend-framework.pot, languages/fr_FR.po) to include the new string and refresh metadata/line references.

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