refactor(design): align typography, colors, and card surfaces to DESIGN.md (#859)

* refactor(design): align surface tokens to DESIGN.md §2

* refactor(design): canonicalize tracked-mono kickers and display rungs

* refactor(design): collapse to five-slot palette and align card surfaces

frontend/src/components/ui/card.tsx

@@ -47,7 +47,7 @@ const CardDescription = React.forwardRef<
 >(({ className, ...props }, ref) => (
   <div
     ref={ref}
-    className={cn("text-sm text-muted-foreground", className)}
+    className={cn("text-sm text-stat-subtitle", className)}
     {...props}
   />
 ))