Update assistant action buttons and AI Chat welcome layout

[astandrik]

Implement the assistant presentation updates from #4448 using the existing assistant integration (design reference).

• Use outlined Diagnostics and Fix buttons while preserving their icons, actions and disabled/selected behavior.
• Keep the AI assistant panel header and replace the illustrated welcome state with AI Chat, the description and four suggestions specified in Implement Healthcheck and AI Chat design review updates #4448.
• Center the short list; let longer content wrap and scroll while the compact input and disclaimer stay visible. Keep the existing populated-chat behavior.
• Verify light/dark themes, keyboard operation, single submission, send/stop/retry/clear and panel resizing.

This work belongs to the application that supplies the assistant buttons and chat through the shared UI's existing extension points. It requires no new assistant API. Compact Healthcheck rows are tracked separately; consume that shared-package update when integrating the complete design.