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>

src/pages/Login.tsx

@@ -4,7 +4,7 @@ import { useNavigate } from 'react-router-dom';
 import { api } from '../api/client';
 
 export const Login: React.FC = () => {
-  const { apiDomain, apiKey, setApiDomain, setApiKey } = useStore();
+  const { apiDomain, apiKey, isMockMode, setApiDomain, setApiKey, setMockMode } = useStore();
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState('');
   const navigate = useNavigate();
@@ -16,10 +16,14 @@ export const Login: React.FC = () => {
 
     try {
       // Basic validation
-      if (!apiDomain || !apiKey) {
+      if (!isMockMode && (!apiDomain || !apiKey)) {
         throw new Error('Please fill all fields');
       }
 
+      if (isMockMode && !apiKey) {
+        setApiKey('mock-session');
+      }
+
       // Check health
       await api.checkHealth();
       
@@ -44,6 +48,7 @@ export const Login: React.FC = () => {
           placeholder="API Domain (e.g. https://api.example.com)"
           value={apiDomain}
           onChange={(e) => setApiDomain(e.target.value)}
+          disabled={isMockMode}
         />
         <input
           type="password"
@@ -51,7 +56,22 @@ export const Login: React.FC = () => {
           placeholder="API Key"
           value={apiKey}
           onChange={(e) => setApiKey(e.target.value)}
+          disabled={isMockMode}
         />
+        
+        <div style={{ marginBottom: '16px', display: 'flex', alignItems: 'center' }}>
+          <input
+            type="checkbox"
+            id="mockMode"
+            checked={isMockMode}
+            onChange={(e) => setMockMode(e.target.checked)}
+            style={{ marginRight: '8px' }}
+          />
+          <label htmlFor="mockMode" style={{ fontSize: '14px', cursor: 'pointer' }}>
+            Enable Mock Mode (UI Only)
+          </label>
+        </div>
+
         {error && (
           <p style={{ color: 'var(--md-sys-color-error)', marginBottom: '16px' }}>
             {error}