✨ 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/pages/projets/[slug].vue

@@ -0,0 +1,19 @@
+<template>
+  <div class="min-h-screen p-8">
+    <h1 class="text-3xl font-narrative text-sky-text">{{ slug }}</h1>
+    <p class="mt-4 text-sky-text/70">{{ $t('pages.projects.description') }}</p>
+  </div>
+</template>
+
+<script setup lang="ts">
+const route = useRoute()
+const slug = computed(() => route.params.slug as string)
+
+const { setPageMeta } = useSeo()
+const { t } = useI18n()
+
+setPageMeta({
+  title: slug.value,
+  description: t('pages.projects.description'),
+})
+</script>

frontend/app/pages/projets/index.vue

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+<template>
+  <div class="min-h-screen p-8">
+    <h1 class="text-3xl font-narrative text-sky-text">{{ $t('pages.projects.title') }}</h1>
+    <p class="mt-4 text-sky-text/70">{{ $t('pages.projects.description') }}</p>
+  </div>
+</template>
+
+<script setup lang="ts">
+const { setPageMeta } = useSeo()
+const { t } = useI18n()
+
+setPageMeta({
+  title: t('pages.projects.title'),
+  description: t('pages.projects.description'),
+})
+</script>