fix: build on debian slim instead of alpine

Alpine + libc6-compat still lost the fight with Tailwind v4's prebuilt
oxide / lightningcss / rolldown bindings on the Portainer host. Debian
slim (glibc-native) is the known-good base for this stack. The serve
stage stays nginx:alpine, so the deployed image size is unchanged —
only the discarded build stage grew.

Also flip the home-page navbar from floating to the standard layout.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/pages/HomePage.vue

@@ -6,7 +6,7 @@ import Button from '@/design-system/components/Button.vue'
 
 <template>
   <div class="min-h-screen bg-surface">
-    <Navbar variant="brand" layout="floating" :cart-count="0" />
+    <Navbar variant="brand" layout="standard" :cart-count="0" />
 
     <section class="mx-auto max-w-4xl px-6 py-28 text-center">
       <p class="eyebrow mb-4">Scaffolding</p>