Create Dashboard layout wrapper component

[yusuftomilola]

Description:
Create a DashboardLayout.tsx component that wraps dashboard pages with the sidebar and main content area. Handles responsive layout where sidebar collapses to a mobile drawer on small screens.

Acceptance Criteria:

• Sidebar on the left, main content area on the right
• Responsive: sidebar becomes mobile drawer on small screens
• Mobile header with menu toggle button
• Accepts children prop for page content

File(s): frontend/components/dashboard/DashboardLayout.tsx (new)