feat: first version of the interface, integrate Tailwind CSS and update routing

- Add Tailwind CSS dependencies and configure Vite to use Tailwind
- Implement routing with React Router for Landing and Dashboard pages
- Remove unused App.css file and refactor App component to utilize new structure
- Update global styles in index.css to incorporate Tailwind's utility classes

frontend/.env.example

@@ -0,0 +1,7 @@
+# Base URL of the FastAPI backend (no trailing slash).
+# Example for local dev: http://localhost:8000
+VITE_API_URL=http://localhost:8000
+
+# Set to "true" while the backend is not yet implemented.
+# All API calls will be served by src/services/mocks.js instead of `fetch`.
+VITE_USE_MOCKS=true