The activity pipeline was already complete end to end — Activity type, store,
WS broadcast, server validation, presence relay, and an ActivityCard used by
four list surfaces — but the profile card rendered none of it, which is the
'Listening to Spotify' block the design calls for.
Add ProfileActivity: richer than ActivityCard because the card has room for
artwork, track and artist, so it reads details/state/assets. All optional, so
it degrades to the bare name that today's process-based detector supplies.
Also scheme-check activity image assets server-side. activity.url was already
restricted to http(s) but assets.largeImage/smallImage were only length-checked
— an asymmetry that was harmless while nothing rendered them, and is not once
they become <img src>: a client could point them at a host it controls and
harvest the IP of everyone opening that profile.
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