feat(newsletter, checkout): add consent links, unify trust-note styling, optimize images

Every email-signup form (Home/newsletter Newsletter section, /newsletter
hero, /challenge, the Newsletter overlay) now links "Datenschutzerklärung"
to /datenschutz next to its consent checkbox, opened in a new tab so a
partially filled form isn't lost. Checkout gets an equivalent AGB +
Datenschutzerklärung note under the order button, and the Versand
modal's Widerrufsbelehrung link also opens in a new tab.

Unified the "Keine Werbung. Jederzeit abbestellbar." trust note (icon +
#888 copy) across all newsletter forms, matching /challenge's existing
style instead of each form having its own wording/color.

Also converted the remaining raw <img> tags in these files to next/image
per the no-img-element lint rule (bandwidth/LCP).

app/checkout/components/CheckoutContent.tsx

@@ -293,7 +293,9 @@ export function CheckoutContent({
                 <span className="flex-1 text-body-sm text-text-primary">{method.title}</span>
                 <span className="flex items-center gap-2 shrink-0">
                   {method.icons.map((icon, i) => (
-                    <img key={i} alt="" src={icon} className="h-5 w-auto" />
+                    <div key={i} className="relative h-5 w-8 shrink-0">
+                      <Image src={icon} alt="" fill sizes="32px" className="object-contain" />
+                    </div>
                   ))}
                 </span>
               </label>
@@ -307,8 +309,19 @@ export function CheckoutContent({
               Jetzt kaufen (zahlungspflichtig)
             </Link>
 
+            <p className="text-label text-text-muted text-center w-full">
+              Mit dem Kauf akzeptierst du unsere{" "}
+              <Link href="/agb" target="_blank" rel="noopener noreferrer" className="underline hover:text-brand">
+                AGB
+              </Link>{" "}
+              und die{" "}
+              <Link href="/datenschutz" target="_blank" rel="noopener noreferrer" className="underline hover:text-brand">
+                Datenschutzerklärung
+              </Link>.
+            </p>
+
             <div className="flex gap-2 items-center justify-center w-full">
-              <img alt="" src="/icon-lock.svg" className="w-3.5 h-4" />
+              <Image alt="" src="/icon-lock.svg" width={14} height={16} className="w-3.5 h-4" />
               <span className="text-body-sm text-text-muted">Sichere SSL-Verschlüsselung</span>
             </div>
           </Reveal>
@@ -398,7 +411,7 @@ export function CheckoutContent({
           <div className="flex flex-col gap-4 items-start w-full">
             {trustBadges.map((b) => (
               <div key={b.id} className="flex gap-3 items-start w-full">
-                <img alt="" src={b.icon} className="size-5 shrink-0 object-contain mt-0.5" />
+                <Image alt="" src={b.icon} width={20} height={20} className="size-5 shrink-0 object-contain mt-0.5" />
                 <div className="flex-1 flex flex-col gap-0.5">
                   <p className="text-body-sm font-semibold text-text-primary">{b.title}</p>
                   <p className="text-label text-text-muted">{b.description}</p>
@@ -412,7 +425,7 @@ export function CheckoutContent({
               which puts the divider under the icon — the two aren't meant
               to be identical, see project notes on this pattern). */}
           <div className="bg-bg-muted flex gap-5 items-start p-6 rounded-md w-full">
-            <img alt="" src="/icon-envelope-hint.png" className="h-8 w-10 shrink-0 object-contain" />
+            <Image alt="" src="/icon-envelope-hint.png" width={40} height={32} className="h-8 w-10 shrink-0 object-contain" />
             <div className="flex flex-col gap-2 items-start flex-1 min-w-0 text-text-primary">
               {/* Conditional on the (unchecked-by-default) consent
                   checkbox above, not stated as a given — a pre-ticked