fix(frontend): move dashboard to /dashboard route

The dashboard page was created at (authenticated)/page.tsx which would
serve the same route as [locale]/page.tsx (the public landing page).
Next.js doesn't allow route groups to override parent pages.

Changes:
- Move dashboard page to (authenticated)/dashboard/page.tsx
- Update Header nav links to point to /dashboard
- Update AppBreadcrumbs home link to /dashboard
- Update E2E tests to navigate to /dashboard

Now authenticated users should navigate to /dashboard for their homepage,
while /en serves the public landing page for unauthenticated users.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/app/[locale]/(authenticated)/dashboard/page.tsx

@@ -0,0 +1,23 @@
+/**
+ * Dashboard Page
+ *
+ * Main authenticated homepage showing:
+ * - Quick stats overview
+ * - Recent projects
+ * - Pending approvals
+ * - Real-time activity feed
+ *
+ * @see Issue #53
+ */
+
+import { Metadata } from 'next';
+import { Dashboard } from '@/components/dashboard';
+
+export const metadata: Metadata = {
+  title: 'Dashboard',
+  description: 'Overview of your projects, agents, and activity',
+};
+
+export default function DashboardPage() {
+  return <Dashboard />;
+}