🎨 Palette: [UX improvement] Add ARIA labels to icon-only buttons

[thirdeyenation]

💡 What:
Added aria-label attributes to icon-only action buttons across the index.html, tasks-list.html, and file-browser.html components. Also added aria-hidden="true" to the inner Material Symbols <span> tags.

🎯 Why:
Icon-only buttons rely entirely on visual context or hover tooltips (title attributes) for meaning. Screen reader users without visual context might hear "vertical align top" or "delete sweep" (the raw ligature text) instead of the button's intended action. This change ensures screen readers correctly announce the button's purpose (e.g., "Scroll to top", "Clear task chat") while ignoring the decorative icon font ligatures, significantly improving the app's overall accessibility.

📸 Before/After:
Visuals unchanged, but accessibility tree updated:

• Before: Screen reader reads "keyboard arrow up button" or "delete sweep button".
• After: Screen reader reads "Previous user message button" or "Clear task chat button".

♿ Accessibility:

• Added descriptive aria-labels to 9 separate action buttons (chat navigation, task management, file browser actions) mapping directly to their existing hover tooltips.
• Applied aria-hidden="true" to the inner <span class="material-symbols-outlined"> elements, adhering to standard practices for icon-font ligatures to prevent confusing and unhelpful screen-reader announcements.

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PR created automatically by Jules for task 9312942335078782361 started by @thirdeyenation

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