- 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.
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