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

.prettierrc.json

@@ -0,0 +1,15 @@
+{
+  "printWidth": 100,
+  "tabWidth": 2,
+  "useTabs": false,
+  "semi": false,
+  "singleQuote": false,
+  "quoteProps": "as-needed",
+  "jsxSingleQuote": false,
+  "trailingComma": "es5",
+  "bracketSpacing": true,
+  "bracketSameLine": true,
+  "arrowParens": "always",
+  "htmlWhitespaceSensitivity": "css",
+  "endOfLine": "lf"
+}