Hardcoded Navigation and Routing Improvements

### Summary :
The `Header.js` component uses hardcoded navigation links and the `Main.js` container uses an older version of `react-router-dom` (Switch). Modernizing the routing and making the header data-driven would improve maintainability.

**File Path :** `components/header/Header.js` and `containers/Main.js`

**Code Improvement :**

```
// Suggested change in Header.js to use a configuration file
import { greeting } from "../../portfolio.js"; // Assume data is centralized

// Instead of hardcoded

Start by reading components/header/Header.js and containers/Main.js to compare the current hardcoded links and routing usage with the issue's proposed direction. Check the project's installed routing version and existing portfolio configuration before deciding how navigation data should be represented. Done means navigation is configuration-driven, routing uses the modern API, and the portfolio still builds and navigates correctly.

Written by the indexing model from the issue text.