[Task] Align sent-bubble attachment rendering with the composer chip design

[Astro-Han]

What needs to be done

PR #1247 (issue #1242) unified the composer's attachment entry points into one chip design: file chips with icon + filename + size, image chips with thumbnails, a bare close-small remove glyph, and a capability warning triangle, all sharing one right-edge rail inside the chip.

The sent message bubble still renders attachments with the older design (packages/ui/src/components/message-file.ts classification + user-message.tsx renderAttachments): square tiles with a filename overlay, different sizing, no shared visual language with the composer.

Bring the bubble's attachment row in line with the composer chip design so an attachment looks the same before and after sending.

Scope notes

• Rendering only; the wire format and the metadata.attachment chip classification from [Feature] Unified attachment chips in the desktop composer #1242 stay as they are.
• The bubble row is display-only (no remove button), so the chip visual carries over minus interactive affordances; click-to-expand for images should be kept.
• Follow docs/DESIGN.md and verify with the existing attachment-chips snap target plus a bubble-side snap or manual walk.

Context

• Composer chip implementation: packages/app/src/components/prompt-input/attachment-chips.tsx
• Bubble rendering: packages/ui/src/components/message-part/user-message.tsx
• Raised during PR feat(app): unify desktop attachment chips with image thumbnails #1247 manual review; deliberately kept out of that PR's boundary.