fix(ui): card density is a Card prop, and compact is compact on desktop

Nineteen cards spelled the compact tier by hand as gap-2 py-3 md:py-4,
which left the base md:gap-6 in place: a compact card opened a 24 px
gap between header and body on a desktop. Card takes density="compact"
now and pins the gap on both breakpoints; every hand-written compact
card, the priority card, the document card and the sample briefing
move onto it, and a test refuses the class list on a Card.

The precision guard on the steps page now reads rendered text only:
the page header's grid template (minmax(0,1fr)) matched its pattern.

src/components/documents/document-card.tsx

@@ -217,8 +217,9 @@ export function DocumentCard({
       data-slot="document-card"
       data-variant={variant}
       data-document-id={document.id}
+      density="compact"
       className={cn(
-        "group relative h-full gap-2 py-3 transition-shadow md:py-4",
+        "group relative h-full transition-shadow",
         "hover:shadow-sm",
         highlighted && "ring-primary ring-2",
       )}