[BUG]: Header navigation links disappear on small screens without a responsive menu

### Bug Description

The application header navigation is not responsive on smaller screen widths.

When the viewport width is reduced to around 550px or below, the header navigation links such as Overview, Repositories, Contributors, Network, Analytics, and Governance are hidden. However, no menu button, hamburger icon, or other navigation option is provided to access these links.

This makes the main navigation inaccessible on smaller screens.

Current Behaviour

At approximately 550px viewport width:

* The header navigation links disappear.
* No responsive menu or navigation toggle is displayed.
* Users cannot access the hidden navigation links from the header.

### Steps to Reproduce

* Open the OrgExplorer application.
* Navigate to any page where the main header is visible.
* Resize the browser window to approximately 550px width or smaller.
* Observe the header navigation.
* The navigation links disappear.
* Observe that there is no menu button or other option to access the hidden navigation links.

### Logs and Screenshots

No console errors or application logs are produced.

Current Behaviour

At a wider viewport, the navigation links are visible in the header.

At approximately 550px width, the navigation links are hidden while no alternative navigation control is available.

The attached screenshots demonstrate the header at different viewport widths and the disappearance of the navigation links on the smaller viewport.

Expected Behaviour

The header should provide a responsive navigation experience on smaller screens.

When the viewport becomes too narrow to display the navigation links:

* The navigation links should be hidden from the desktop layout.
* A menu/hamburger button should be displayed in the header.
* Clicking the menu button should open a responsive navigation menu.
* The menu should provide access to the same navigation links:
* Overview
* Repositories
* Contributors
* Network
* Analytics
* Governance
* The menu should be accessible without requiring the user to increase the browser width.

### Environment Details

Environment Details

* OS: Windows 11
* Browser: Google Chrome
* Application: OrgExplorer
* Viewport: Approximately 550px or smaller
* Area: Global application header/navigation
* No console errors observed

### Impact

Medium - Feature works but has issues

Screenshort

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Start at the global application header/navigation entry point and reproduce the issue at approximately 550px in Chrome. Verify that the responsive menu provides access to Overview, Repositories, Contributors, Network, Analytics, and Governance, and confirm these links remain accessible at narrow widths.

索引モデルが issue の本文から書いたものです。