Replace the placeholder Screenshots section with a hero voice/video shot
and a curated 6-shot grid (chat, screen-share controls, spaces, federated
DMs, social discovery, federation admin), linking to a new
docs/screenshots.md gallery of all 14 shots grouped by feature.
Images are optimized to 1600px WebP (~1.2MB total vs ~45MB raw); raw
originals in demo-pictures/ are gitignored.
