feat: Add mock mode for UI-only development

- Created mock API implementation in src/api/mock.ts
- Added isMockMode toggle to useStore
- Implemented switching between real and mock API in client.ts
- Added Mock Mode toggle to Login page
- Added 'npm run dev:mock' command

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

.env.example

@@ -1 +1,2 @@
 VITE_API_DOMAIN=http://localhost:3000
+VITE_MOCK_MODE=false