feat(frontend): add Dashboard page and components for #53

Implement the main dashboard homepage with:
- WelcomeHeader: Personalized greeting with user name
- DashboardQuickStats: Stats cards for projects, agents, issues, approvals
- RecentProjects: Dynamic grid showing 3-6 recent projects
- PendingApprovals: Action-required approvals section
- EmptyState: Onboarding experience for new users
- useDashboard hook: Mock data fetching with React Query

The dashboard serves as the authenticated homepage at /(authenticated)/
and provides quick access to all project management features.

frontend/src/components/dashboard/WelcomeHeader.tsx

@@ -0,0 +1,55 @@
+/**
+ * WelcomeHeader Component
+ *
+ * Displays a personalized welcome message for the dashboard.
+ *
+ * @see Issue #53
+ */
+
+'use client';
+
+import { Plus } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+import { Link } from '@/lib/i18n/routing';
+import { useAuth } from '@/lib/auth/AuthContext';
+
+export interface WelcomeHeaderProps {
+  /** Additional CSS classes */
+  className?: string;
+}
+
+export function WelcomeHeader({ className }: WelcomeHeaderProps) {
+  const { user } = useAuth();
+
+  // Get first name for greeting
+  const firstName = user?.first_name || user?.email?.split('@')[0] || 'there';
+
+  // Get time-based greeting
+  const getGreeting = () => {
+    const hour = new Date().getHours();
+    if (hour < 12) return 'Good morning';
+    if (hour < 18) return 'Good afternoon';
+    return 'Good evening';
+  };
+
+  return (
+    <div className={className}>
+      <div className="flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between">
+        <div>
+          <h1 className="text-3xl font-bold tracking-tight">
+            {getGreeting()}, {firstName}
+          </h1>
+          <p className="mt-1 text-muted-foreground">
+            Here&apos;s what&apos;s happening with your projects today.
+          </p>
+        </div>
+        <Button asChild>
+          <Link href="/projects/new">
+            <Plus className="mr-2 h-4 w-4" />
+            Create Project
+          </Link>
+        </Button>
+      </div>
+    </div>
+  );
+}