fix(mobile): image/video aspect-ratio so media doesn't crop/letterbox (N2)

MImage/MVideo pinned AttachmentBox to a fixed height computed for a 400px-wide
layout. The box width is responsive (maxWidth:100%) but the height was frozen,
so on a phone the box narrows below 400px while keeping desktop height ->
images crop (object-fit:cover) and videos letterbox (object-fit:contain).

Drive the box by `aspect-ratio: w/h` when intrinsic dimensions are known, so the
height tracks the responsive width. On desktop the box stays 400px wide, so the
aspect-ratio yields the identical height (algebraically 400*h/w =
scaleYDimension(w,400,h)) — pixel-identical. Falls back to the fixed height when
dimensions are unknown; the 48px floor and 600px cap are preserved.

Uses the same pattern already shipped in this codebase (TwitchThumbnailWrapper,
GalleryTile). Two review passes, one empirically measuring the rendered image in
Chromium: desktop unchanged, narrow widths keep correct aspect, no collapse.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/message/MsgTypeRenderers.tsx

@@ -417,6 +417,22 @@ type RenderImageContentProps = {
   markedAsSpoiler?: boolean;
   spoilerReason?: string;
 };
+// Media frame sizing. When intrinsic width/height are known, drive the box by
+// aspect-ratio so its height tracks the responsive (maxWidth:100%) width — a
+// fixed pixel height computed for a 400px-wide layout otherwise crops (images,
+// object-fit:cover) or letterboxes (videos, object-fit:contain) on phones where
+// the box narrows below 400px. On desktop the box stays 400px wide, so the
+// aspect-ratio yields the identical height. Falls back to the fixed height when
+// dimensions are unknown.
+const attachmentMediaStyle = (
+  w: number | undefined,
+  h: number | undefined,
+  fallbackHeight: number,
+): CSSProperties =>
+  w && h
+    ? { aspectRatio: `${w} / ${h}`, minHeight: toRem(48) }
+    : { height: toRem(fallbackHeight < 48 ? 48 : fallbackHeight) };
+
 type MImageProps = {
   content: IImageContent;
   renderImageContent: (props: RenderImageContentProps) => ReactNode;
@@ -432,11 +448,7 @@ export function MImage({ content, renderImageContent, outlined }: MImageProps) {
 
   return (
     <Attachment outlined={outlined}>
-      <AttachmentBox
-        style={{
-          height: toRem(height < 48 ? 48 : height),
-        }}
-      >
+      <AttachmentBox style={attachmentMediaStyle(imgInfo?.w, imgInfo?.h, height)}>
         {renderImageContent({
           body: content.body || 'Image',
           info: imgInfo,
@@ -498,11 +510,7 @@ export function MVideo({ content, renderAsFile, renderVideoContent, outlined }:
           }
         />
       </AttachmentHeader>
-      <AttachmentBox
-        style={{
-          height: toRem(height < 48 ? 48 : height),
-        }}
-      >
+      <AttachmentBox style={attachmentMediaStyle(videoInfo.w, videoInfo.h, height)}>
         {renderVideoContent({
           body: content.body || 'Video',
           info: videoInfo,