style: one spacing, title and skeleton language across the app

Icons inside buttons drop their manual margins in favour of the
button's built-in gap (two hundred call sites), form field stacks settle
on one spacing step, card titles on exactly two sizes, inline field
errors on one text size, and a dozen hand-rolled loading pulses move
onto the shared skeleton primitive while keeping their silhouettes. The
intake dialog wording joins the rest of the app on 'übersprungen', and
the UI guidelines now document the two deliberate footer and
touch-target systems.

src/components/data-list/selection-action-bar.tsx

@@ -88,9 +88,9 @@ export function SelectionActionBar({
             disabled={isDeleting}
           >
             {isDeleting ? (
-              <Loader2 className="mr-2 h-4 w-4 animate-spin motion-reduce:animate-none" />
+              <Loader2 className="h-4 w-4 animate-spin motion-reduce:animate-none" />
             ) : (
-              <Trash2 className="mr-2 h-4 w-4" />
+              <Trash2 className="h-4 w-4" />
             )}
             {t("dataList.deleteN", { count: String(count) })}
           </Button>