Fix more mobile responsive issues: Hero, arrows, cart button, checkmarks, modal

- Newsletter card icon: fixed an aspect-ratio distortion bug (w-16 maps
  to this project's fluid --spacing-16, which floors to 40px below
  768px, paired with a fixed 55px height — squished the icon on
  mobile). Same root cause existed in NewsletterModal's envelope icon;
  that one's now just hidden below md: instead per feedback.
- Hero: subtitle sized down and CTA checkmark icon scaled to match
  below lg:, social proof text always wraps below the avatars there
  instead of only when it doesn't fit.
- Werkzeuge connector arrows (todo-cards/newsletter/challenge
  HowItWorks): object-contain added — the SVG has
  preserveAspectRatio="none" and was stretching to fill the square
  mobile box instead of keeping its thin-arrow shape.
- Challenge bottom CTA: icon now stacks above the copy, centered, below
  lg: to match the Home Newsletter card's pattern.
- Homepage Werkzeuge icons: smaller below md: to match the (fluid-floor)
  text size next to them.
- Blog detail "Passend dazu" card: the fixed w-[19rem] title column plus
  "Entdecken" sharing its row overflowed on mobile; stacked below sm:
  instead, with a little top margin on "Entdecken".
- Todo-Karten checklist checkmarks: recolored brand-orange (icon-check
  .svg's fill lives in an internal CSS var that can't be overridden from
  outside an <img>-loaded SVG, so switched to an inline SVG) and
  top-aligned instead of vertically centered.
- AddToCartButton: added whitespace-nowrap to the stacked-label grid
  (used to reserve button width across all possible label texts) — one
  of those labels wrapping to two lines on a narrow w-full button was
  inflating the row height for whichever label is actually showing,
  leaving a tall empty gap under "Ausverkauft".

app/newsletter/components/HowItWorks.tsx

@@ -76,12 +76,16 @@ export function HowItWorks() {
               // technique as Challenge's step connector and todo-cards'
               // identical HowItWorks, not just the same icon asset.
               <div className="flex items-center justify-center shrink-0 md:mt-[1.625rem]">
+                {/* object-contain — see todo-cards/HowItWorks.tsx's own
+                    comment on this same fix: the SVG's preserveAspectRatio
+                    ="none" stretched it to fill the square mobile box
+                    instead of keeping its thin-arrow ratio. */}
                 <Image
                   alt=""
                   src="/icon-arrow-connector.svg"
                   width={24}
                   height={24}
-                  className="w-6 h-6 rotate-90 md:w-10 md:h-3 md:rotate-0"
+                  className="w-6 h-6 object-contain rotate-90 md:w-10 md:h-3 md:rotate-0"
                 />
               </div>
             )}