🎨 Palette: UX and Accessibility improvements for copy buttons and icons

[anaxifty]

This PR introduces several micro-UX and accessibility improvements focused on providing clear feedback for user actions and improving the experience for assistive technology users.

💡 What: The UX enhancement added

1. Clipboard Feedback: A new copyFeedback utility in js/utils.js provides immediate inline visual and ARIA feedback when a "Copy" button is clicked. It temporarily updates the button text and label to "Copied!" (or a custom message) while preserving any icons.
2. Accessibility (A11y) Sweep: Added aria-label attributes to all primary icon-only buttons in the interface, including sidebar toggles, attachment/voice inputs, and chat send/stop buttons. Decorative SVGs are now hidden from screen readers.

🎯 Why: The user problem it solves

• Users previously only had toast notifications for copy actions, which are often far from the point of interaction. Inline feedback is more direct and reassuring.
• Icon-only buttons without labels are inaccessible to screen reader users.

♿ Accessibility: Any a11y improvements made

• Added descriptive aria-label to 6+ icon-only buttons.
• Synchronized aria-label updates during feedback cycles.
• Marked decorative icons as aria-hidden.
• Improved input labeling for the main chat textarea.

Verified with Playwright functional testing.

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PR created automatically by Jules for task 14235495152825937064 started by @anaxifty

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