🎨 Palette: Amélioration de l'accessibilité du modal d'importation GitHub

[bobdivx]

💡 What: Ajout d'attributs ARIA et d'états de focus visibles au composant GithubImportModal.
🎯 Why: Améliorer l'accessibilité pour les utilisateurs de lecteurs d'écran et la navigation au clavier.
📸 Before/After: Les boutons ont maintenant des états de focus visibles, le bouton de fermeture a un label ARIA, et la fenêtre modale indique correctement son rôle au navigateur.
♿ Accessibility:

• Ajout de `aria-hidden="true"` aux SVG décoratifs.
• Ajout de `role="dialog"`, `aria-modal="true"`, et `aria-labelledby` au conteneur de la modale.
• Ajout de `aria-label` au bouton de fermeture.

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PR created automatically by Jules for task 10116504176181020469 started by @bobdivx

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