🎨 Palette: Add ARIA role="alert" to chat warning banners

[docxology]

💡 What: Added role="alert" attribute to the dynamically generated error warning banners when Ollama is offline or unavailable.
🎯 Why: Without this ARIA attribute, screen readers will not announce these important system warnings automatically to visually impaired users, resulting in a critical accessibility gap.
📸 Before/After: The visual style hasn't changed.
♿ Accessibility: Improves accessibility for screen readers by explicitly marking warning banners as alert dialogs using the standard ARIA attribute role="alert".

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PR created automatically by Jules for task 2663056762803685614 started by @docxology

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