🎨 Palette: Accessibility sweep, search persistence, and interactive feedback

[anaxifty]

🎨 Palette UX Enhancement Suite

This PR implements a collection of micro-UX improvements focused on accessibility, interaction polish, and intuitive state management:

1. Accessibility Sweep: Added aria-label to all icon-only buttons (Sidebar, Header, Chat, Canvas, and IDE) and aria-hidden="true" to their decorative SVGs. Search inputs now have descriptive labels for screen reader users.
2. Search Persistence & Polish:
   • renderSidebar now reads the current search value if not provided, preventing filter loss when pinning or deleting conversations.
   • Added an accessible empty state ("No matches found") with role="status" and aria-live="polite".
   • Starting a "New Chat" now clears the sidebar search to ensure the fresh state is immediately visible.
3. Standardized Status Feedback: Model search results in the selector and rewrite picker now correctly announce "No results" to assistive technologies.
4. Interactive Copy Feedback: The "Copy" buttons in the chat and code blocks now provide immediate inline feedback by temporarily changing text to "Copied!" and synchronizing the aria-label for a high-confidence interaction.

All changes are under 50 lines per module and follow existing design patterns. Verified via Playwright and manual inspection.

Critical learnings recorded in .Jules/palette.md.

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PR created automatically by Jules for task 913974092693088900 started by @anaxifty

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