🎨 Palette: Add aria-label to share button

[kuasar-mknd]

💡 What: Added an aria-label attribute to the share button (#share-btn) in src/pages/[slug].astro.
🎯 Why: To improve accessibility for screen readers, as the button only contained an icon without any descriptive text. This ensures it matches the behavior already present in src/pages/project/[slug].astro.
📸 Before/After: Visuals verified via Playwright screenshot.
♿ Accessibility: The button is now properly labeled as 'Partager cet article' for screen reader users.

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PR created automatically by Jules for task 16649436036586350486 started by @kuasar-mknd

[google-labs-jules]

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