feat: dashboard overview page with stats, activity, and up-next (HQ-21)

- New Dashboard page: task stats grid, active tasks, up-next queue, recent activity, recently completed
- Dashboard is now the home page (/)
- Sidebar: Dashboard → Queue → Projects → Chat
- Queue page defaults redirect to / instead of /queue

frontend/src/App.tsx

@@ -1,5 +1,6 @@
 import { BrowserRouter, Routes, Route, Navigate } from "react-router-dom";
 import { DashboardLayout } from "./components/DashboardLayout";
+import { DashboardPage } from "./pages/DashboardPage";
 import { QueuePage } from "./pages/QueuePage";
 import { ChatPage } from "./pages/ChatPage";
 import { ProjectsPage } from "./pages/ProjectsPage";
@@ -12,11 +13,12 @@ function AuthenticatedApp() {
     <BrowserRouter>
       <Routes>
         <Route element={<DashboardLayout />}>
+          <Route path="/" element={<DashboardPage />} />
           <Route path="/queue" element={<QueuePage />} />
           <Route path="/projects" element={<ProjectsPage />} />
           <Route path="/chat" element={<ChatPage />} />
           <Route path="/admin" element={<AdminPage />} />
-          <Route path="*" element={<Navigate to="/queue" replace />} />
+          <Route path="*" element={<Navigate to="/" replace />} />
         </Route>
       </Routes>
     </BrowserRouter>