docs: Add comprehensive multi-child implementation plan

Added detailed implementation plan covering:
- Frontend: Dynamic UI, child selector, bulk activity logging, comparison analytics
- Backend: Bulk operations, multi-child queries, family statistics
- AI/Voice: Child name detection, context building, clarification flows
- Database: Schema enhancements, user preferences, bulk operation tracking
- State management, API enhancements, real-time sync updates
- Testing strategy: Unit, integration, and E2E tests
- Migration plan with feature flags for phased rollout
- Performance optimizations: Caching, indexes, code splitting

Also includes:
- Security fixes for multi-family data leakage in analytics pages
- ParentFlow branding updates
- Activity tracking navigation improvements
- Backend DTO and error handling fixes

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

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

maternal-web/app/analytics/page.tsx

@@ -40,6 +40,7 @@ import PredictionsCard from '@/components/features/analytics/PredictionsCard';
 import GrowthSpurtAlert from '@/components/features/analytics/GrowthSpurtAlert';
 import WeeklyReportCard from '@/components/features/analytics/WeeklyReportCard';
 import MonthlyReportCard from '@/components/features/analytics/MonthlyReportCard';
+import { useAuth } from '@/lib/auth/AuthContext';
 
 interface TabPanelProps {
   children?: React.ReactNode;
@@ -65,6 +66,7 @@ function TabPanel(props: TabPanelProps) {
 
 export default function AnalyticsPage() {
   const theme = useTheme();
+  const { user } = useAuth();
   const [children, setChildren] = useState<Child[]>([]);
   const [selectedChildId, setSelectedChildId] = useState<string>('');
   const [tabValue, setTabValue] = useState(0);
@@ -74,27 +76,54 @@ export default function AnalyticsPage() {
   const [insightsLoading, setInsightsLoading] = useState(false);
   const [predictionsLoading, setPredictionsLoading] = useState(false);
   const [days, setDays] = useState<number>(7);
+  const [error, setError] = useState<string>('');
+
+  const familyId = user?.families?.[0]?.familyId;
 
   useEffect(() => {
-    loadChildren();
-  }, []);
-
-  useEffect(() => {
-    if (selectedChildId) {
-      loadInsights();
-      loadPredictions();
+    if (familyId) {
+      loadChildren();
     }
-  }, [selectedChildId, days]);
+  }, [familyId]);
+
+  useEffect(() => {
+    if (selectedChildId && children.length > 0) {
+      // Validate that selectedChildId belongs to current user's children
+      const childExists = children.some(child => child.id === selectedChildId);
+      if (childExists) {
+        loadInsights();
+        loadPredictions();
+      } else {
+        console.warn('[AnalyticsPage] Selected child not found in user\'s children, resetting');
+        setSelectedChildId(children[0].id);
+        setError('Selected child not found. Showing data for your first child.');
+      }
+    }
+  }, [selectedChildId, days, children]);
 
   const loadChildren = async () => {
+    if (!familyId) {
+      setLoading(false);
+      setError('No family found');
+      return;
+    }
+
     try {
-      const data = await childrenApi.getChildren();
+      console.log('[AnalyticsPage] Loading children for familyId:', familyId);
+      const data = await childrenApi.getChildren(familyId);
+      console.log('[AnalyticsPage] Loaded children:', data);
       setChildren(data);
-      if (data.length > 0 && !selectedChildId) {
-        setSelectedChildId(data[0].id);
+
+      if (data.length > 0) {
+        const existingChildStillValid = data.some(child => child.id === selectedChildId);
+        if (!selectedChildId || !existingChildStillValid) {
+          setSelectedChildId(data[0].id);
+        }
       }
+      setError('');
     } catch (error) {
-      console.error('Failed to load children:', error);
+      console.error('[AnalyticsPage] Failed to load children:', error);
+      setError('Failed to load children');
     } finally {
       setLoading(false);
     }