Replaces #21279 and #21282 with one clean PR from `main`.
Generalizes the settings primary-bar / secondary-bar card chrome to the
record index, record show and standalone pages via a shared
`PageCardLayout` + `PageCardHeader` (the side panel sits as a sibling of
the content card), and applies the new flat design direction: square
corners on the card, side panel and loading skeletons.
Iterating toward the new design (Figma node 102282-221623); the
confirmed direction and the explicit "remove rounded corners" change are
in, remaining designer specifics to follow.
Félix Malfait
bfefcd3755