Add demo mode support with MSW integration and documentation

- Integrated Mock Service Worker (MSW) for frontend-only demo mode, allowing API call interception without requiring a backend.
- Added `DemoModeBanner` component to indicate active demo mode and display demo credentials.
- Enhanced configuration with `DEMO_MODE` flag and demo credentials for user and admin access.
- Updated ESLint configuration to exclude MSW-related files from linting and coverage.
- Created comprehensive `DEMO_MODE.md` documentation for setup and usage guidelines, including deployment instructions and troubleshooting.
- Updated package dependencies to include MSW and related libraries.

frontend/src/mocks/browser.ts

@@ -0,0 +1,71 @@
+/**
+ * MSW Browser Setup
+ *
+ * Configures Mock Service Worker for browser environment.
+ * This intercepts network requests at the network layer, making it transparent to the app.
+ */
+
+import { setupWorker } from 'msw/browser';
+import { handlers } from './handlers';
+
+/**
+ * Create MSW worker with all handlers
+ * This worker intercepts fetch/XHR requests in the browser
+ */
+export const worker = setupWorker(...handlers);
+
+/**
+ * Check if MSW should be started
+ * Only runs when ALL conditions are met:
+ * - In browser (not SSR)
+ * - NOT in Jest test environment
+ * - NOT in Playwright E2E tests
+ * - Demo mode explicitly enabled
+ */
+function shouldStartMSW(): boolean {
+  if (typeof window === 'undefined') {
+    return false; // SSR, skip
+  }
+
+  // Skip Jest unit tests
+  if (process.env.NODE_ENV === 'test') {
+    return false;
+  }
+
+  // Skip Playwright E2E tests (uses your existing __PLAYWRIGHT_TEST__ flag)
+  if ((window as any).__PLAYWRIGHT_TEST__) {
+    return false;
+  }
+
+  // Only start if demo mode is explicitly enabled
+  return process.env.NEXT_PUBLIC_DEMO_MODE === 'true';
+}
+
+/**
+ * Start MSW for demo mode
+ * SAFE: Will not interfere with unit tests or E2E tests
+ */
+export async function startMockServiceWorker() {
+  if (!shouldStartMSW()) {
+    // Silently skip - this is normal for dev/test environments
+    return;
+  }
+
+  try {
+    await worker.start({
+      onUnhandledRequest: 'warn', // Warn about unmocked requests
+      serviceWorker: {
+        url: '/mockServiceWorker.js',
+      },
+    });
+
+    console.log('%c[MSW] Demo Mode Active', 'color: #00bfa5; font-weight: bold;');
+    console.log('[MSW] All API calls are mocked (no backend required)');
+    console.log('[MSW] Demo credentials:');
+    console.log('  Regular user: demo@example.com / DemoPass123');
+    console.log('  Admin user: admin@example.com / AdminPass123');
+  } catch (error) {
+    console.error('[MSW] Failed to start Mock Service Worker:', error);
+    console.error('[MSW] Demo mode will not work correctly');
+  }
+}