🎨 Palette: Enhance Search Accessibility and UI Feedback

[anaxifty]

This PR implements a series of micro-UX and accessibility improvements to the AI Studio interface.

💡 What

• Accessibility Audit: Added aria-label attributes to all critical icon-only buttons (Send, Stop, Attach, IDE actions, etc.) and marked decorative SVGs with aria-hidden="true".
• Search Feedback: Implemented accessible 'No matches found' messages for the sidebar conversation search and model selectors using role="status" and aria-live="polite".
• State Persistence: Refined the sidebar rendering logic to preserve active search filters when pinning/unpinning conversations.
• Workflow Polish: The 'New Chat' action now programmatically clears the search input to ensure the newly created conversation is immediately visible.

🎯 Why

• Screen reader users previously received ambiguous feedback from icon-only buttons.
• Dynamic UI updates like empty search results were not being announced to assistive technologies.
• Users lost their search context when interacting with conversation items (e.g., pinning), leading to a disjointed experience.

♿ Accessibility

• Full coverage of aria-label for primary interaction buttons.
• Compliance with ARIA Live Regions for dynamic search status updates.

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PR created automatically by Jules for task 1821149724468423277 started by @anaxifty

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