style(chat): UI: add mobile layout for chat compose actions

[junyiz]

• Stack chat compose row vertically on mobile (max-width: 640px)
• Change action buttons to vertical layout with full width
• Improve mobile UX for send and session control buttons

Greptile Overview

Greptile Summary

This PR updates ui/src/styles/chat/layout.css to improve the chat compose area on small screens (@media (max-width: 640px)). On mobile it switches .chat-compose__row and .chat-compose__actions from horizontal to vertical flex layout, adds smaller gaps, and makes compose action buttons full-width to better fit narrow viewports. These styles layer on top of the existing desktop compose/controls flex layout rules defined earlier in the same stylesheet.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• The change is confined to a small, well-scoped CSS media-query adjustment for mobile layout, with no functional logic changes and no evidence of breaking selectors or invalid CSS in the diff reviewed.
• No files require special attention

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