🎨 Palette: Improve accessibility of icon-only navigation and task buttons

[thirdeyenation]

💡 What: Added aria-label attributes to icon-only buttons in the chat navigation and task list, and aria-hidden="true" to their internal Material Symbol spans.
🎯 Why: Screen readers rely on aria-labels to announce icon-only buttons, and without aria-hidden="true" on the span, they read out the literal text of the ligature (e.g., "keyboard arrow down") instead of or in addition to the button's purpose, confusing users.
📸 Before/After: Screen readers previously announced "keyboard_arrow_up", now they announce "Previous user message".
♿ Accessibility: Improves WCAG compliance for non-text content and keyboard/screen-reader navigation.

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PR created automatically by Jules for task 1641240210788330423 started by @thirdeyenation

[google-labs-jules]

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