feat(02-03): add per-route SEO metadata and JSON-LD to all page stubs

- useSeoMeta() with localized title/description/og tags on all 6 pages
- Homepage JSON-LD with Person + ProfessionalService schema
- og:image absolute URL on every page
- Stub templates with max-w-7xl wrapper and h1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/pages/about.vue

@@ -0,0 +1,21 @@
+<script setup lang="ts">
+const { t } = useI18n()
+
+useSeoMeta({
+  title: () => t('seo.about.title'),
+  description: () => t('seo.about.description'),
+  ogTitle: () => t('seo.about.title'),
+  ogDescription: () => t('seo.about.description'),
+  ogImage: 'https://killiandalcin.fr/og-image.png',
+  ogImageWidth: 1200,
+  ogImageHeight: 630,
+  ogType: 'website',
+})
+</script>
+
+<template>
+  <div class="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 py-12">
+    <h1 class="text-2xl font-bold">{{ t('nav.about') }}</h1>
+    <p class="text-gray-600 dark:text-gray-400 mt-4">Phase 3 content placeholder</p>
+  </div>
+</template>