ui : fit wide image attachments to the message width
Attachment thumbnails used a fixed height with w-auto, so a wide image kept its aspect-driven width and, being flex-shrink-0 in a right-aligned bubble, overflowed to the left of the message row.
Cap the thumbnail with max-height and max-width instead of a fixed height so it scales down proportionally, and let it shrink outside the single-row carousel.
Assisted-by: pi:deepseek-ai/DeepSeek-V4.1-Flash
