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/page.tsx

@@ -124,7 +124,7 @@ export default function HomePage() {
     { icon: <Hotel />, label: t('quickActions.sleep'), color: theme.palette.secondary.main, path: '/track/sleep' },
     { icon: <BabyChangingStation />, label: t('quickActions.diaper'), color: theme.palette.warning.main, path: '/track/diaper' },
     { icon: <MedicalServices />, label: t('quickActions.medical'), color: theme.palette.error.main, path: '/track/medicine' },
-    { icon: <Insights />, label: t('quickActions.activities'), color: theme.palette.success.main, path: '/activities' },
+    { icon: <Insights />, label: t('quickActions.activities'), color: theme.palette.success.main, path: '/track/activity' },
     { icon: <SmartToy />, label: t('quickActions.aiAssistant'), color: theme.palette.info.main, path: '/ai-assistant' },
   ];