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

env.d.ts

@@ -0,0 +1,10 @@
+/// <reference types="vite/client" />
+
+import 'vue-router'
+
+declare module 'vue-router' {
+  interface RouteMeta {
+    requiresAuth?: boolean
+    guestOnly?: boolean
+  }
+}