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/DashboardView.vue

@@ -0,0 +1,10 @@
+<script setup lang="ts"></script>
+
+<template>
+  <div>
+    <h1 class="text-2xl font-bold">Dashboard</h1>
+    <p class="mt-2 text-muted">
+      This page uses the sidebar layout. Replace this content with your own dashboard widgets.
+    </p>
+  </div>
+</template>