feat: add mobile responsive support (768px breakpoint)

- Add CSS media queries for all sections: sidebar overlay, navbar,
  calendar, kanban, dashboard, list view, drawer, modal, reports,
  team tasks, members
- Add hamburger menu button (hidden on desktop, visible on mobile)
- Add sidebar slide-in overlay with backdrop for mobile
- Auto-close sidebar on navigation for mobile UX
- Login card, drawer, and modal go full-width on mobile
- Dashboard stats grid collapses to 2-column on mobile
- Report charts stack to single column on mobile

src/NavBars.tsx

@@ -8,11 +8,13 @@ interface TopNavbarProps {
     searchQuery: string;
     onSearch: (q: string) => void;
     onNewTask: () => void;
+    onOpenSidebar: () => void;
 }
 
-export function TopNavbar({ title, filterUser, onFilterChange, searchQuery, onSearch, onNewTask }: TopNavbarProps) {
+export function TopNavbar({ title, filterUser, onFilterChange, searchQuery, onSearch, onNewTask, onOpenSidebar }: TopNavbarProps) {
     return (
         <div className="top-navbar">
+            <button className="hamburger-btn" onClick={onOpenSidebar}>☰</button>
             <span className="navbar-title">{title}</span>
             <div className="navbar-search">
                 <span className="navbar-search-icon">🔍</span>