Responsive Navigation & Mobile Menu

[Luluameh]

Source: FRONTEND_ISSUES.md (Issue 17)

Background
The current layout needs to be fully responsive for mobile devices.
Goal
Enhance the layout and navigation for smaller screens.
Acceptance Criteria

• Hamburger menu on mobile.
• Bottom navigation bar for dashboard routes on mobile.
  Key Files
  src/components/layout/Navbar.tsx, src/components/layout/MobileMenu.tsx

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