fix(newsletter): stack inline footer form on mobile

Email field + submit button stacked on mobile (button full width below the
input) and side by side from sm up. The long "Newsletter abonnieren" button
was squeezing the email field on narrow screens.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/internal/NewsletterInlineFormComponent.svelte

@@ -137,7 +137,7 @@
         <input name="company" type="text" tabindex="-1" autocomplete="off" bind:value={company} />
       </div>
 
-      <div class="flex gap-2">
+      <div class="flex flex-col gap-2 sm:flex-row">
         <input
           type="email"
           name="email"
@@ -151,7 +151,7 @@
         <button
           type="submit"
           disabled={formState === "pending"}
-          class="shrink-0 rounded-sm bg-wald-600 px-4 py-2 text-sm font-semibold text-stein-0 transition-colors hover:bg-wald-500 disabled:opacity-60"
+          class="rounded-sm bg-wald-600 px-4 py-2 text-sm font-semibold text-stein-0 transition-colors hover:bg-wald-500 disabled:opacity-60 sm:shrink-0"
         >
           {formState === "pending" ? t(T.newsletter_submitting) : t(T.newsletter_submit)}
         </button>