✨ Add layouts, routing, transitions & pages (Story 1.4)

Default layout with sticky AppHeader (nav, LanguageSwitcher, mobile hamburger),
AppFooter with social links. Minimal layout for express mode. 7 placeholder pages
with localized EN routes. Page transitions (fade+slide), prefers-reduced-motion
support, custom scroll behavior, error.vue, useSeo composable, SVG favicon.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/app/composables/useSeo.ts

@@ -0,0 +1,38 @@
+interface SeoOptions {
+  title: string
+  description?: string
+  image?: string
+  url?: string
+}
+
+export const useSeo = () => {
+  const config = useRuntimeConfig()
+  const route = useRoute()
+  const { locale } = useI18n()
+
+  const setPageMeta = (options: SeoOptions) => {
+    const siteUrl = config.public.siteUrl as string || 'https://skycel.fr'
+    const fullUrl = options.url || `${siteUrl}${route.fullPath}`
+    const imageUrl = options.image || `${siteUrl}/og-image.jpg`
+
+    useHead({
+      title: options.title,
+    })
+
+    useSeoMeta({
+      title: options.title,
+      description: options.description,
+      ogTitle: options.title,
+      ogDescription: options.description,
+      ogImage: imageUrl,
+      ogUrl: fullUrl,
+      ogLocale: locale.value === 'fr' ? 'fr_FR' : 'en_US',
+      twitterCard: 'summary_large_image',
+      twitterTitle: options.title,
+      twitterDescription: options.description,
+      twitterImage: imageUrl,
+    })
+  }
+
+  return { setPageMeta }
+}