refactor: enhance semantic HTML by adding ARIA roles to header, footer, and main elements

[cedric07]

Le RGAA recommande de conserver ces attributs même s'ils disposent de lien d'accès rapides : https://accessibilite.numerique.gouv.fr/methode/glossaire/#landmarks et https://accessibilite.numerique.gouv.fr/methode/criteres-et-tests/#12.6

C'était également un retour de Temesis lors d'un audit.

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Note

Low Risk
Pure markup/accessibility changes (adds roles) with no impact on business logic or data handling; minimal risk aside from potential CSS/JS selectors that might rely on attributes.

Overview
Improves accessibility semantics by explicitly adding ARIA landmark role attributes to the theme layout.

header.php now sets role="banner" on the header, role="navigation" on the main menu nav, and role="main" on the main content container; footer.php adds role="contentinfo" to the footer.

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