[UI/UX] Implement OS Selection Dropdown in Header

[chojuninengu]

Goal

Allow users to switch between Operating Systems (macOS, Linux) at any time from the persistent Navbar.

Requirements

• Add a dropdown selector to Navbar.tsx.
• Synchronize the selection with the global os state in the store.
• Removing the need to 'Start Over' just to check commands for a different platform.

Impact

Improves the user experience by making the platform feel like a workspace rather than a multi-step wizard.