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/eslint.config.mjs

@@ -17,6 +17,8 @@ export default [
       'dist/**',
       'coverage/**',
       'src/lib/api/generated/**',
+      'src/mocks/**', // MSW mock data (demo mode only, not production code)
+      'public/mockServiceWorker.js', // Auto-generated by MSW
       '*.gen.ts',
       '*.gen.tsx',
       'next-env.d.ts', // Auto-generated by Next.js