Swap the fake gallery tiles' gradient stand-ins for 13 rights-clean
generated WebP stills (512px, ~25 KB each). Search demo now returns
thematically matched results — beach/dunes for the filename query, sunset
and water for /s, landscapes for /t — so the show-don't-tell steps look
like a real library. tileGradient retained for the abstract Explore and
Timeline mini-previews.
