Resize chat image previews naturally

[zzj3720]

Summary

• size chat image previews from intrinsic dimensions when they are within bounds
• constrain oversized previews to max 640x520 and upscale tiny previews to at least 160x120 while preserving aspect ratio
• apply the same sizing helper to attachment images and markdown images, and remove the old fixed thumbnail/max-height clamps

Validation

• cd web && yarn typecheck
• cd web && yarn lint
• cd web && yarn test
• cd web && yarn build:embedded (passes with existing rspack asset-size warnings)
• Manual/non-GUI sizing sanity: 400x300 => 400x300, 1436x1240 => 602x520, 80x60 => 160x120, 1024x1024 => 520x520. Full Safari visual smoke was blocked in this broker runtime by macOS launch error -10810.