Fix responsive message images

Constrain image attachments to their parent while preserving their aspect ratio.

src/app/components/message/attachment/Attachment.css.ts

@@ -8,7 +8,7 @@ export const Attachment = recipe({
     color: color.SurfaceVariant.OnContainer,
     borderRadius: config.radii.R400,
     overflow: 'hidden',
-    maxWidth: toRem(400),
+    maxWidth: `min(100%, ${toRem(400)})`,
   },
   variants: {
     outlined: {
@@ -33,7 +33,7 @@ export const AttachmentHeader = style({
 export const AttachmentBox = style([
   DefaultReset,
   {
-    maxWidth: toRem(400),
+    maxWidth: `min(100%, ${toRem(400)})`,
     maxHeight: toRem(400),
     overflow: 'hidden',
   },