Refactor Layout and Styling in UserMenu, AppHeader, and Product Pages

- Adjusted button margin in UserMenu.vue for better alignment.
- Updated container classes in AppHeader.vue to use max-width for improved responsiveness.
- Modified max-width settings in product detail page to enhance layout consistency across different states (loading, error, product display).

app/pages/products/[id].vue

@@ -116,7 +116,7 @@ const handleAddToCart = async () => {
   <NuxtLayout name="default">
     <div class="min-h-screen px-4 py-12 md:px-6 lg:px-8">
       <!-- Back Button -->
-      <div class="mx-auto mb-8 max-w-container-narrow">
+      <div class="mx-auto max-w-container-wide">
         <NuxtLink to="/products"
           class="inline-flex items-center gap-2 text-white/80 transition-colors hover:text-white">
           <ArrowLeft :size="20" />
@@ -125,14 +125,14 @@ const handleAddToCart = async () => {
       </div>
 
       <!-- Loading State -->
-      <div v-if="pending" class="mx-auto max-w-container-narrow text-center">
+      <div v-if="pending" class="mx-auto max-w-4xl text-center">
         <div class="card-glass inline-block">
           <p class="text-white/80">Produkt wird geladen...</p>
         </div>
       </div>
 
       <!-- Error State (404 or other errors) -->
-      <div v-else-if="error" class="mx-auto max-w-container-narrow">
+      <div v-else-if="error" class="mx-auto max-w-4xl">
         <div class="card-glass border-red/50 bg-red/10">
           <h2 class="mb-2 text-2xl font-semibold text-red">
             {{ error.statusCode === 404 ? 'Produkt nicht gefunden' : 'Fehler beim Laden' }}
@@ -153,7 +153,7 @@ const handleAddToCart = async () => {
       </div>
 
       <!-- Product Detail -->
-      <div v-else-if="product" class="mx-auto max-w-container-narrow">
+      <div v-else-if="product" class="mx-auto mt-12 max-w-container-wide">
         <div class="overflow-hidden rounded-2xl border border-white/20 bg-white/10 shadow-glass backdrop-blur-lg">
           <!-- Product Image (no padding, flush with top) -->
           <div class="relative aspect-[16/9] w-full overflow-hidden bg-purple-dark">