Fix several mobile responsive issues across Hero, Newsletter, cart, and Challenge

- Hero CTA: back to a single line below lg: (wrapping put the icon
  beside two lines and looked broken), sized down instead so the full
  phrase fits.
- Hero heading: dropped the forced <br> after "darf" at true mobile
  widths, kept it only for the sm-lg tablet range it was added for.
- Hero social proof: avatars+text now wrap and center instead of
  cramming onto one row (flex-wrap + justify-center, no fixed
  breakpoint needed).
- Newsletter card icon: stacked above the copy and centered below md:
  instead of squeezed into a row beside it.
- TrustRow: left-aligned below md: instead of centered (shop, cart, and
  every other page using this component).
- Cart line item image: full width below sm: instead of a fixed 150px
  square.
- Challenge EmailCapture (top + bottom CTA, same shared component):
  input and button stack full-width below sm: instead of squeezing
  into one row.
- Challenge steps: icon above text and centered at every breakpoint
  (previously only from lg: up), connector arrows centered to match.

app/challenge/components/EmailCapture.tsx

@@ -22,7 +22,13 @@ export function EmailCapture({ buttonLabel = "Challenge starten" }: { buttonLabe
 
   return (
     <form onSubmit={handleSubmit} className="flex flex-col gap-2 w-full">
-      <div className="flex gap-3 w-full">
+      {/* Stacked full-width below sm: — side by side, the button's own
+          content width plus the input's min-w-0 squeeze left it cramped
+          on a narrow phone. Default align-items: stretch in flex-col
+          mode is what makes both the input and the button (shrink-0,
+          fixed to its label's width) fill the row once stacked, no
+          explicit w-full needed on either. */}
+      <div className="flex flex-col sm:flex-row gap-3 w-full">
         <input
           ref={emailRef}
           type="email"