FE016: Add collapsible sidebar to Employer Panel

[shahbeebawan]

Overview
Implemented a collapsible navigation sidebar for the SecureShift Employer Panel.

Changes

• Added a reusable Sidebar component.
• Added collapse/expand functionality.
• Displays navigation icons when collapsed.
• Displays navigation labels when expanded.
• Preserved existing Employer Panel navigation routes.
• Added active navigation state highlighting.
• Updated the protected layout so main content resizes alongside the sidebar.
• Added responsive styling for laptop and tablet screen sizes.

Testing

• Verified the sidebar component compiles successfully.
• Verified collapse/expand logic and responsive layout implementation in code.
• Full end-to-end testing of protected Employer Panel pages could not be completed locally because the backend authentication/login flow is currently unavailable in my local environment.

Notes
No backend changes are included in this PR.