🎨 Palette: Make mobile theme toggle accessible

[ItsHarta]

💡 What: Replaced the mobile theme toggle <Icon> with an accessible semantic <button> in AppLayout.astro.
🎯 Why: To allow keyboard navigation for screen readers and keyboard users (focusability) while introducing ARIA properties to communicate the state.
📸 Before/After: The icon design matches the previous design pixel-for-pixel by keeping structural layout identical but wrapping the <Icon> element inside a flex container <button>.
♿ Accessibility: Added aria-label, native title tooltip, aria-expanded true/false toggling logic to JS, and focus-visible keyboard styling.
🌗 Dark mode: Both light and dark variants retain correct styling, adjusting icon states depending on visibility.

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PR created automatically by Jules for task 402579579082886906 started by @ItsHarta

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