Add admin UX improvements, constants refactor, and comprehensive tests

- Introduced constants for admin hooks: `STATS_FETCH_LIMIT`, `DEFAULT_PAGE_LIMIT`, and `STATS_REFETCH_INTERVAL` to enhance readability and maintainability.
- Updated query guards to ensure data fetching is restricted to superusers.
- Enhanced accessibility across admin components by adding `aria-hidden` attributes and improving focus-visible styles.
- Simplified `useAdminStats`, `useAdminUsers`, and `useAdminOrganizations` with shared constants.
- Added 403 Forbidden page with proper structure, styling, and tests.
- Implemented new tests for admin hooks, DashboardStats, AdminLayout, and ForbiddenPage for better coverage.

frontend/tests/app/forbidden/page.test.tsx

@@ -0,0 +1,66 @@
+/**
+ * Tests for 403 Forbidden Page
+ * Verifies rendering of access forbidden message and navigation
+ */
+
+import { render, screen } from '@testing-library/react';
+import ForbiddenPage from '@/app/forbidden/page';
+
+describe('ForbiddenPage', () => {
+  it('renders page heading', () => {
+    render(<ForbiddenPage />);
+
+    expect(
+      screen.getByRole('heading', { name: /403 - Access Forbidden/i })
+    ).toBeInTheDocument();
+  });
+
+  it('renders permission denied message', () => {
+    render(<ForbiddenPage />);
+
+    expect(
+      screen.getByText(/You don't have permission to access this resource/)
+    ).toBeInTheDocument();
+  });
+
+  it('renders admin privileges message', () => {
+    render(<ForbiddenPage />);
+
+    expect(
+      screen.getByText(/This page requires administrator privileges/)
+    ).toBeInTheDocument();
+  });
+
+  it('renders link to dashboard', () => {
+    render(<ForbiddenPage />);
+
+    const dashboardLink = screen.getByRole('link', {
+      name: /Go to Dashboard/i,
+    });
+    expect(dashboardLink).toBeInTheDocument();
+    expect(dashboardLink).toHaveAttribute('href', '/dashboard');
+  });
+
+  it('renders link to home', () => {
+    render(<ForbiddenPage />);
+
+    const homeLink = screen.getByRole('link', { name: /Go to Home/i });
+    expect(homeLink).toBeInTheDocument();
+    expect(homeLink).toHaveAttribute('href', '/');
+  });
+
+  it('renders shield alert icon with aria-hidden', () => {
+    const { container } = render(<ForbiddenPage />);
+
+    const icon = container.querySelector('[aria-hidden="true"]');
+    expect(icon).toBeInTheDocument();
+  });
+
+  it('renders with proper container structure', () => {
+    const { container } = render(<ForbiddenPage />);
+
+    const containerDiv = container.querySelector('.container');
+    expect(containerDiv).toBeInTheDocument();
+    expect(containerDiv).toHaveClass('mx-auto', 'px-6', 'py-16');
+  });
+});