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/App.tsx

@@ -34,6 +34,7 @@ export default function App() {
   const [calView, setCalView] = useState('month');
   const [filterUser, setFilterUser] = useState<string | null>(null);
   const [searchQuery, setSearchQuery] = useState('');
+  const [sidebarOpen, setSidebarOpen] = useState(false);
   const [quickAddDay, setQuickAddDay] = useState<{ date: string; rect: { top: number; left: number } } | null>(null);
 
   if (!currentUser) return <LoginPage onLogin={u => { setCurrentUser(u); setActivePage('calendar'); setActiveView('calendar'); }} />;
@@ -41,6 +42,7 @@ export default function App() {
   const handleNavigate = (page: string) => {
     setActivePage(page);
     if (VIEW_PAGES.includes(page)) setActiveView(page);
+    setSidebarOpen(false);
   };
 
   const handleViewChange = (view: string) => {
@@ -88,10 +90,12 @@ export default function App() {
   return (
     <div className="app-shell">
       <TopNavbar title={pageTitle} filterUser={filterUser} onFilterChange={setFilterUser}
-        searchQuery={searchQuery} onSearch={setSearchQuery} onNewTask={handleNewTask} />
+        searchQuery={searchQuery} onSearch={setSearchQuery} onNewTask={handleNewTask}
+        onOpenSidebar={() => setSidebarOpen(true)} />
       <div className="app-body">
         <Sidebar currentUser={currentUser} activePage={activePage} onNavigate={handleNavigate}
-          onSignOut={() => { setCurrentUser(null); setActivePage('calendar'); setActiveView('calendar'); }} />
+          onSignOut={() => { setCurrentUser(null); setActivePage('calendar'); setActiveView('calendar'); setSidebarOpen(false); }}
+          isOpen={sidebarOpen} onClose={() => setSidebarOpen(false)} />
         <div className="main-content">
           {displayPage === 'calendar' && (
             <CalendarView tasks={tasks} currentUser={currentUser} calMonth={calMonth} calView={calView}