fix(CQ-WEB-10): Add loading/error states to pages with mock data

Convert tasks, calendar, and dashboard pages from synchronous mock data
to async loading pattern with useState/useEffect. Each page now shows a
loading state via child components while data loads, and displays a
PDA-friendly amber-styled message with a retry button if loading fails.

This prepares these pages for real API integration by establishing the
async data flow pattern. Child components (TaskList, Calendar, dashboard
widgets) already handled isLoading props — now the pages actually use them.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/app/(authenticated)/page.tsx

@@ -1,3 +1,6 @@
+"use client";
+
+import { useState, useEffect } from "react";
 import type { ReactElement } from "react";
 import { RecentTasksWidget } from "@/components/dashboard/RecentTasksWidget";
 import { UpcomingEventsWidget } from "@/components/dashboard/UpcomingEventsWidget";
@@ -5,43 +8,71 @@ import { QuickCaptureWidget } from "@/components/dashboard/QuickCaptureWidget";
 import { DomainOverviewWidget } from "@/components/dashboard/DomainOverviewWidget";
 import { mockTasks } from "@/lib/api/tasks";
 import { mockEvents } from "@/lib/api/events";
+import type { Task, Event } from "@mosaic/shared";
 
 export default function DashboardPage(): ReactElement {
-  // TODO: Replace with real API call when backend is ready
-  // const { data: tasks, isLoading: tasksLoading } = useQuery({
-  //   queryKey: ["tasks"],
-  //   queryFn: fetchTasks,
-  // });
-  // const { data: events, isLoading: eventsLoading } = useQuery({
-  //   queryKey: ["events"],
-  //   queryFn: fetchEvents,
-  // });
+  const [tasks, setTasks] = useState<Task[]>([]);
+  const [events, setEvents] = useState<Event[]>([]);
+  const [isLoading, setIsLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
 
-  const tasks = mockTasks;
-  const events = mockEvents;
-  const tasksLoading = false;
-  const eventsLoading = false;
+  useEffect(() => {
+    void loadDashboardData();
+  }, []);
+
+  async function loadDashboardData(): Promise<void> {
+    setIsLoading(true);
+    setError(null);
+
+    try {
+      // TODO: Replace with real API calls when backend is ready
+      // const [tasksData, eventsData] = await Promise.all([fetchTasks(), fetchEvents()]);
+      await new Promise((resolve) => setTimeout(resolve, 300));
+      setTasks(mockTasks);
+      setEvents(mockEvents);
+    } catch (err) {
+      setError(
+        err instanceof Error
+          ? err.message
+          : "We had trouble loading your dashboard. Please try again when you're ready."
+      );
+    } finally {
+      setIsLoading(false);
+    }
+  }
 
   return (
     <main className="container mx-auto px-4 py-8">
       <div className="mb-8">
         <h1 className="text-3xl font-bold text-gray-900">Dashboard</h1>
-        <p className="text-gray-600 mt-2">Welcome back! Here's your overview</p>
+        <p className="text-gray-600 mt-2">Welcome back! Here&apos;s your overview</p>
       </div>
 
-      <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
-        {/* Top row: Domain Overview and Quick Capture */}
-        <div className="lg:col-span-2">
-          <DomainOverviewWidget tasks={tasks} isLoading={tasksLoading} />
+      {error !== null ? (
+        <div className="rounded-lg border border-amber-200 bg-amber-50 p-6 text-center">
+          <p className="text-amber-800">{error}</p>
+          <button
+            onClick={() => void loadDashboardData()}
+            className="mt-4 rounded-md bg-amber-600 px-4 py-2 text-sm font-medium text-white hover:bg-amber-700 transition-colors"
+          >
+            Try again
+          </button>
         </div>
+      ) : (
+        <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
+          {/* Top row: Domain Overview and Quick Capture */}
+          <div className="lg:col-span-2">
+            <DomainOverviewWidget tasks={tasks} isLoading={isLoading} />
+          </div>
 
-        <RecentTasksWidget tasks={tasks} isLoading={tasksLoading} />
-        <UpcomingEventsWidget events={events} isLoading={eventsLoading} />
+          <RecentTasksWidget tasks={tasks} isLoading={isLoading} />
+          <UpcomingEventsWidget events={events} isLoading={isLoading} />
 
-        <div className="lg:col-span-2">
-          <QuickCaptureWidget />
+          <div className="lg:col-span-2">
+            <QuickCaptureWidget />
+          </div>
         </div>
-      </div>
+      )}
     </main>
   );
 }