feat: render desktop latest section as 3-column masonry

- Matches Figma design (file uHDZkVHjAp7BXDKQKB0PM4, node 4367-11405).
- Mobile keeps the existing 5-post single column unchanged.
- Desktop (md+) renders all 12 latest posts in a CSS-columns masonry
  with break-inside-avoid so each card's height stays content-driven.
- Adds an optional 'fluid' prop to MessageBubble that drops the
  standalone-feed max-widths so bubbles fill the masonry column. The
  /browse stream keeps the default non-fluid widths.

src/components/messageStream/MessageBubble.tsx

@@ -23,7 +23,15 @@ export function pickBubble(post: Post): BubbleComponent {
   return FileDocBubble;
 }
 
-export function MessageBubble({ post }: { post: Post }) {
+export function MessageBubble({
+  post,
+  fluid = false,
+}: {
+  post: Post;
+  /** When true, fill the parent container instead of applying the standalone
+   *  feed max-widths. Used by the desktop 3-column masonry on the home page. */
+  fluid?: boolean;
+}) {
   const Bubble = pickBubble(post);
   const isVisual =
     Bubble === AlbumBubble ||
@@ -34,7 +42,11 @@ export function MessageBubble({ post }: { post: Post }) {
   return (
     <div
       id={`post-${post.id}`}
-      className="mx-auto w-full max-w-[358px] scroll-mt-[82px] md:max-w-[680px] md:scroll-mt-[98px] lg:max-w-[900px] xl:max-w-[1120px]"
+      className={
+        fluid
+          ? "w-full scroll-mt-[82px] md:scroll-mt-[98px]"
+          : "mx-auto w-full max-w-[358px] scroll-mt-[82px] md:max-w-[680px] md:scroll-mt-[98px] lg:max-w-[900px] xl:max-w-[1120px]"
+      }
     >
       <article
         className={`relative w-full overflow-hidden rounded-2xl bg-[#272632] text-left shadow-sm ${