Nudge checkmark/login icon vertical alignment

- Checklist checkmarks (Challenge, todo-cards, newsletter): mt-0.5 -> mt-1,
  centering against the first line's glyphs instead of sitting flush at
  the very top edge.
- Navbar login icon: -translate-y-0.5 — its round head vs. wide shoulders
  read as optically bottom-heavy next to the cart icon despite both
  having a mathematically centered bounding box.

app/challenge/page.tsx

@@ -78,7 +78,7 @@ function IconCheckCircle() {
 
 function Check() {
   return (
-    <svg width="18" height="18" viewBox="0 0 18 18" fill="none" className="shrink-0 mt-0.5">
+    <svg width="18" height="18" viewBox="0 0 18 18" fill="none" className="shrink-0 mt-1">
       <path d="M3 9.5l4 4L15 4" stroke="#f6a701" strokeWidth="2.2" strokeLinecap="round" strokeLinejoin="round" />
     </svg>
   );