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/store/useStore.ts

@@ -4,8 +4,10 @@ import { persist } from 'zustand/middleware';
 interface AppState {
   apiDomain: string;
   apiKey: string;
+  isMockMode: boolean;
   setApiDomain: (domain: string) => void;
   setApiKey: (key: string) => void;
+  setMockMode: (isMock: boolean) => void;
   reset: () => void;
 }
 
@@ -14,8 +16,10 @@ export const useStore = create<AppState>()(
     (set) => ({
       apiDomain: import.meta.env.VITE_API_DOMAIN || '',
       apiKey: '',
+      isMockMode: import.meta.env.VITE_MOCK_MODE === 'true',
       setApiDomain: (apiDomain) => set({ apiDomain }),
       setApiKey: (apiKey) => set({ apiKey }),
+      setMockMode: (isMockMode) => set({ isMockMode }),
       reset: () => set({ apiKey: '' }),
     }),
     {