Improve chat attachments UI

[andrii-i]

Problem

Chat message attachments (introduced in #248) currently display as plain text chips with an oversized close button. The attachment bar takes up excessive vertical space relative to the input area and doesn't visually match JupyterLab's file tab or tag styling conventions.

Proposed Solution

Redesign the attachments UI. Some suggestions:

• Use compact file-tag style chips consistent with JupyterLab's visual language
• Reduce vertical footprint of the attachment bar
• Use a subtler dismiss affordance (smaller icon or hover-to-reveal)
• Use Material UI directly or as a reference for high bar of UI UX quality

[nakul-py]

This is what we have before

[brichet]

Thanks @andrii-i for opening this.
It is probably a side effect of #330.

I opened #376