feat: Add responsive sidebar collapse toggle for desktop (#834)

[devjayy43]

Summary

Implements a minimal responsive sidebar collapse feature for desktop users.

Closes #834

Features

• Desktop Collapse Toggle: New collapse/expand button in sidebar header (desktop only)
• Icon-Only Collapsed View: Sidebar collapses from w-64 to w-20, showing only navigation icons
• Smooth Animations: CSS transitions for collapse/expand with duration-200
• Hidden Labels: Logo text, navigation labels, and user info hidden when collapsed
• Tooltip Support: Navigation items show full name on hover via title attribute
• Dynamic Content Margins: Main content area adjusts margins dynamically (lg:ml-64 ↔ lg:ml-20)
• Accessibility: Proper aria-labels, aria-expanded, and focus management
• Mobile Preserved: Mobile toggle (Menu/X) remains unchanged, collapse is desktop-only

Changes

• New: frontend/contexts/SidebarContext.tsx - Context for managing global sidebar state
• Updated: frontend/components/layout/Sidebar.tsx - Added collapse toggle button and conditional rendering
• Updated: frontend/components/layout/DashboardAuthGuard.tsx - Wrapped with SidebarProvider and adjusted margins

Testing

• Desktop: Click collapse button in sidebar header to toggle collapsed state
• Desktop: Verify sidebar transitions smoothly between w-64 and w-20
• Desktop: Verify icons are centered and labels hidden when collapsed
• Desktop: Hover over navigation icons to see tooltip with full name
• Desktop: Verify main content margin adjusts dynamically
• Mobile: Verify menu button still works as expected
• Mobile: Verify no collapse button appears on mobile

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