fix: Add mock user for development to resolve children fetch issue

Added mock test user to auth slice initialState to provide familyId
for children fetching in development environment.

Issue: Tracking pages couldn't fetch children because state.auth.user
was null, resulting in no familyId for API calls.

Solution: Mock user with test familyId in development mode only.

TODO: Implement proper authentication flow in production.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

maternal-web/store/slices/authSlice.ts

@@ -15,10 +15,19 @@ export interface AuthState {
   loading: boolean;
 }
 
+// Mock user for development (TODO: Remove in production and implement real auth)
+const MOCK_USER: User = {
+  id: 'user_test123',
+  email: 'test@maternal.app',
+  name: 'Test User',
+  familyId: 'fam_test123',
+};
+
 const initialState: AuthState = {
-  user: null,
+  // Use mock user in development if no real user is stored
+  user: process.env.NODE_ENV === 'development' ? MOCK_USER : null,
   token: null,
-  isAuthenticated: false,
+  isAuthenticated: process.env.NODE_ENV === 'development',
   loading: false,
 };