🎨 Palette: Add custom visual tooltip to Hamburger menu button

[kuasar-mknd]

💡 What: Added a custom visual tooltip to the mobile HamburgerButton component that appears on hover and focus. It explicitly indicates the state (Menu / Fermer) and provides a discoverable keyboard shortcut (Esc) that appears inline on larger viewports.
🎯 Why: While the button had correct aria-labels for screen readers, visual users on tablet devices or keyboard-navigating users on desktop-resized windows didn't have clear feedback on the button's action or shortcut key.
📸 Before/After: See visual verification screenshots (hamburger_hover_closed.png / open.png).
♿ Accessibility: Added aria-hidden="true" to the visual tooltip so it doesn't duplicate the existing aria-label for screen reader users, keeping the accessible name clean while enhancing visual UI context.

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PR created automatically by Jules for task 6021722608436666779 started by @kuasar-mknd

[google-labs-jules]

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