feat: initialise Vue 3 SPA template

- Add Tailwind CSS v4 with light/dark theme support
- Add layout system (default, sidebar, blank)
- Add mock authentication with Pinia store and route guards
- Add error pages (404, 500) and example views
- Add Vitest with 10 example tests across stores
- Configure Prettier formatting and ESLint linting

src/views/NotFoundView.vue

@@ -0,0 +1,17 @@
+<script setup lang="ts">
+import { RouterLink } from "vue-router"
+</script>
+
+<template>
+  <div
+    class="flex min-h-screen flex-col items-center justify-center gap-4 bg-surface text-foreground p-4">
+    <h1 class="text-6xl font-bold text-muted">404</h1>
+    <p class="text-lg">Page not found</p>
+    <p class="text-sm text-muted">The page you are looking for does not exist or has been moved.</p>
+    <RouterLink
+      to="/"
+      class="mt-4 rounded-md bg-primary px-4 py-2 text-sm text-white hover:bg-primary-hover transition-colours">
+      Back to home
+    </RouterLink>
+  </div>
+</template>