feat(stream): add whitespace below the end-of-feed message

Give the "No more activities." message generous, viewport-scaled space
below it so the end of the feed isn't cramped against the bottom edge.

AI-assisted: Claude Code (Opus 4.8)
Signed-off-by: Jan C. Borchardt <925062+jancborchardt@users.noreply.github.com>

Author: jancborchardt