Add internal styleguide page with HTTP Basic Auth

- Create /internal/styleguide route with protected layout
- Add styleguide.vue layout with auth protection
- Configure internal pages auth credentials in .env.example
- Update nuxt.config for internal routes handling

app/layouts/styleguide.vue

@@ -0,0 +1,10 @@
+<script setup lang="ts">
+// Styleguide layout - minimal, no header/footer
+// Used for internal styleguide page
+</script>
+
+<template>
+  <div class="min-h-screen">
+    <slot />
+  </div>
+</template>