feat: integrate Tailwind CSS and implement dark mode functionality

- Added Tailwind CSS dependencies and configured PostCSS.
- Updated app component to wrap router outlet with a styled div for dark mode support.
- Modified routing to include a default route and added auth guards.
- Implemented dark mode toggle functionality in content component.
- Enhanced login component with improved styling and lifecycle management.
- Created items component with basic structure and routing.
- Added global styles for body and dark mode variants.
- Updated tests for new items component.

Web/.postcssrc.json

@@ -0,0 +1,5 @@
+{
+  "plugins": {
+    "@tailwindcss/postcss": {}
+  }
+}