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/components/AddToCartButton.tsx

@@ -128,7 +128,14 @@ export function AddToCartButton({
             also reserves space for "Ausverkauft"/"Maximale Menge im
             Warenkorb" — the widest of the four wins regardless of which is
             showing. */}
-        <span className="relative grid">
+        {/* whitespace-nowrap — inherited by every stacked span below. On a
+            w-full button (e.g. this page's mobile layout), "Maximale Menge
+            im Warenkorb" is long enough to wrap to two lines without this,
+            and since every stacked span shares the same grid cell, that
+            inflated the row height for whichever text is actually showing
+            too — "Ausverkauft" rendered with a tall empty gap underneath it
+            (fixed 2026-07-24). */}
+        <span className="relative grid whitespace-nowrap">
           <span className="invisible [grid-area:1/1]" aria-hidden="true">
             {label}
           </span>