Move structural breakpoint to 640px, shorten Hero heading

Tablet layout (768-1023px) hit the md: grid switch exactly where the
fluid clamp() tokens were already at their floor, leaving no room to
shrink. Moves the fluid floor and structural breakpoint down together
to sm: (640px) so real tablets always get the fluid, desktop-like
structure; consolidates the ad-hoc md:+lg: patchwork in Hero/About/
Newsletter/Footer/Tools back onto one line, leaving documented lg:
exceptions where content genuinely doesn't fit yet. Also shortens the
Hero heading to a single sentence with no trailing period (the brand's
orange dot already renders one, animated).

app/components/TestimonialsGrid.tsx

@@ -15,7 +15,7 @@ export function TestimonialsGrid({ testimonials }: { testimonials: Testimonial[]
   if (testimonials.length === 0) return null;
 
   return (
-    <section className="w-full bg-bg-base flex flex-col gap-8 items-center py-12 md:py-16 px-[var(--layout-padding-x)]">
+    <section className="w-full bg-bg-base flex flex-col gap-8 items-center py-12 sm:py-16 px-[var(--layout-padding-x)]">
       <div className="max-w-[1600px] mx-auto w-full flex flex-col gap-8 items-center">
         <Reveal
           className="font-semibold text-h-emphasis text-text-primary text-center"
@@ -24,11 +24,11 @@ export function TestimonialsGrid({ testimonials }: { testimonials: Testimonial[]
           Was andere sagen
         </Reveal>
 
-        <RevealGroup className="grid grid-cols-1 md:grid-cols-12 gap-6 md:gap-[var(--layout-grid-gap)] w-full">
+        <RevealGroup className="grid grid-cols-1 sm:grid-cols-12 gap-6 sm:gap-[var(--layout-grid-gap)] w-full">
           {testimonials.map((t) => (
             <RevealItem
               key={t.id}
-              className="group relative md:col-span-4 bg-bg-muted rounded-xl p-6 flex flex-col gap-4 transition-transform duration-300 hover:-translate-y-1"
+              className="group relative sm:col-span-4 bg-bg-muted rounded-xl p-6 flex flex-col gap-4 transition-transform duration-300 hover:-translate-y-1"
             >
               <span
                 aria-hidden