✨ 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/layouts/default.vue

@@ -0,0 +1,11 @@
+<template>
+  <div class="min-h-screen bg-sky-dark text-sky-text flex flex-col">
+    <LayoutAppHeader />
+
+    <main class="flex-1">
+      <slot />
+    </main>
+
+    <LayoutAppFooter />
+  </div>
+</template>