[IMP] base,web: adapt kanban image following image tools changes

Since the images are not guaranteed to be square anymore, the kanban image had
to be adapted to better handle the different image ratios.

Also the placeholder has been moved into the actual image div instead of
being a separate div that acted differently.

The avatar placeholder was white... displayed on top of white. Now it's grey
so we can actually see it.

task-1958000
PR: #31811