🎨 Palette: Accessibility and Usability Improvements for Agent View

[ArnBdev]

This PR implements several micro-UX improvements focused on accessibility and usability in the Agent View:

1. Session Title Editing: Added a pencil icon button next to the session title. This makes the editing feature (previously only available via double-click) more discoverable and accessible. The button is hidden while editing is active.
2. Accessibility Attributes:
   • Added aria-label to icon-only buttons (Settings, New Session, Browse Sessions, Add Context Files, Remove File, Context Toggle).
   • Added aria-expanded state to the context panel toggle button to indicate collapsed/expanded state to screen readers.
   • Added role="textbox" and aria-multiline="true" to the rich text input area for better screen reader support.
3. Testing: Added test/ui/accessibility.test.ts to verify the presence of these new attributes and elements.

These changes follow the "Palette" persona guidelines to enhance the user experience with small but impactful touches.

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PR created automatically by Jules for task 18141383646972268954 started by @ArnBdev

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