#11167: style(chat): UI: add mobile layout for chat compose actions

- 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 <img width="1548" height="414" alt="image" src="https://github.com/user-attachments/assets/e8c1877d-9ac6-443d-bc8b-f786502639b9" /> <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> 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. <h3>Confidence Score: 5/5</h3> - 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 <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->