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>

package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "scripts": {
     "dev": "vite",
+    "dev:mock": "VITE_MOCK_MODE=true vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
     "preview": "vite preview"