feat: add responsive professional information layer

src/components/AppShell.vue

@@ -31,13 +31,13 @@ const canonicalNavigation = computed(() => canonicalSectionIds.map((id) => ({ id
 
 <template>
   <a class="skip-link" href="#portfolio-content">Skip to portfolio content</a>
-  <header>
+  <header class="site-header">
     <h1>{{ content.identity.name }}</h1>
     <p>{{ content.identity.role }}</p>
     <SiteNavigation :items="canonicalNavigation" />
   </header>
 
-  <main id="portfolio-content">
+  <main id="portfolio-content" class="portfolio-main">
     <PortfolioSection id="start" :index="1" :eyebrow="sectionLabel('start').toUpperCase()" :heading="sectionLabel('start').toUpperCase()">
       <p>{{ content.identity.valueProposition }}</p>
     </PortfolioSection>