✨ 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/error.vue

@@ -0,0 +1,40 @@
+<template>
+  <div class="min-h-screen bg-sky-dark text-sky-text flex flex-col items-center justify-center p-8">
+    <h1 class="text-6xl font-bold text-sky-accent mb-4">
+      {{ error?.statusCode || 500 }}
+    </h1>
+
+    <p class="text-xl font-narrative mb-8 text-center max-w-md">
+      {{ errorMessage }}
+    </p>
+
+    <button
+      class="px-6 py-3 bg-sky-accent text-sky-dark font-ui font-semibold rounded-lg hover:opacity-90 transition-opacity"
+      @click="handleError"
+    >
+      {{ $t('common.back_home') }}
+    </button>
+  </div>
+</template>
+
+<script setup lang="ts">
+const props = defineProps<{
+  error: {
+    statusCode: number
+    message: string
+  }
+}>()
+
+const { t } = useI18n()
+
+const errorMessage = computed(() => {
+  if (props.error?.statusCode === 404) {
+    return t('error.404')
+  }
+  return t('error.generic')
+})
+
+function handleError() {
+  clearError({ redirect: '/' })
+}
+</script>