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

.vscode/extensions.json

@@ -0,0 +1,8 @@
+{
+  "recommendations": [
+    "Vue.volar",
+    "dbaeumer.vscode-eslint",
+    "EditorConfig.EditorConfig",
+    "oxc.oxc-vscode"
+  ]
+}