feat: Apply localization to Track and Children pages (Phase 9 - Batch 2)

**Pages Localized:**
- Track main page: Activity selection menu with all tracking options
- Children page: Complete localization including age formatting with pluralization

**Translation Files:**
- Enhanced tracking.json: Added trackActivity, selectActivity, and activities keys
- Created children.json for all 5 languages with comprehensive strings
- Updated i18n config to include children namespace

**Key Features:**
- Localized age calculation with proper pluralization (year/years, month/months)
- All error messages translated
- Gender labels localized
- Properly formatted age display for all languages

**Languages Supported:**
- English, Spanish, French, Portuguese, Chinese (Simplified)

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

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

maternal-web/app/children/page.tsx

@@ -23,8 +23,10 @@ import { childrenApi, Child, CreateChildData } from '@/lib/api/children';
 import { ChildDialog } from '@/components/children/ChildDialog';
 import { DeleteConfirmDialog } from '@/components/children/DeleteConfirmDialog';
 import { motion } from 'framer-motion';
+import { useTranslation } from '@/hooks/useTranslation';
 
 export default function ChildrenPage() {
+  const { t } = useTranslation('children');
   const { user } = useAuth();
   const [children, setChildren] = useState<Child[]>([]);
   const [loading, setLoading] = useState(true);
@@ -43,7 +45,7 @@ export default function ChildrenPage() {
       fetchChildren();
     } else {
       setLoading(false);
-      setError('No family found. Please complete onboarding first.');
+      setError(t('errors.noFamily'));
     }
   }, [familyId]);
 
@@ -57,7 +59,7 @@ export default function ChildrenPage() {
       setChildren(data);
     } catch (err: any) {
       console.error('Failed to fetch children:', err);
-      setError(err.response?.data?.message || 'Failed to load children');
+      setError(err.response?.data?.message || t('errors.loadFailed'));
     } finally {
       setLoading(false);
     }
@@ -80,7 +82,7 @@ export default function ChildrenPage() {
 
   const handleSubmit = async (data: CreateChildData) => {
     if (!familyId) {
-      throw new Error('No family ID found');
+      throw new Error(t('errors.noFamilyId'));
     }
 
     try {
@@ -94,7 +96,7 @@ export default function ChildrenPage() {
       setDialogOpen(false);
     } catch (err: any) {
       console.error('Failed to save child:', err);
-      throw new Error(err.response?.data?.message || 'Failed to save child');
+      throw new Error(err.response?.data?.message || t('errors.saveFailed'));
     } finally {
       setActionLoading(false);
     }
@@ -111,7 +113,7 @@ export default function ChildrenPage() {
       setChildToDelete(null);
     } catch (err: any) {
       console.error('Failed to delete child:', err);
-      setError(err.response?.data?.message || 'Failed to delete child');
+      setError(err.response?.data?.message || t('errors.deleteFailed'));
     } finally {
       setActionLoading(false);
     }
@@ -138,11 +140,11 @@ export default function ChildrenPage() {
     }
 
     if (years === 0) {
-      return `${months} month${months !== 1 ? 's' : ''}`;
+      return `${months} ${months !== 1 ? t('ageFormat.months') : t('ageFormat.month')}`;
     } else if (months === 0) {
-      return `${years} year${years !== 1 ? 's' : ''}`;
+      return `${years} ${years !== 1 ? t('ageFormat.years') : t('ageFormat.year')}`;
     } else {
-      return `${years} year${years !== 1 ? 's' : ''}, ${months} month${months !== 1 ? 's' : ''}`;
+      return `${years} ${years !== 1 ? t('ageFormat.years') : t('ageFormat.year')}, ${months} ${months !== 1 ? t('ageFormat.months') : t('ageFormat.month')}`;
     }
   };
 
@@ -153,10 +155,10 @@ export default function ChildrenPage() {
           <Box sx={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', mb: 4 }}>
             <Box>
               <Typography variant="h4" component="h1" fontWeight="600" gutterBottom>
-                Children
+                {t('title')}
               </Typography>
               <Typography variant="body1" color="text.secondary">
-                Manage your family's children profiles
+                {t('subtitle')}
               </Typography>
             </Box>
             <Button
@@ -165,7 +167,7 @@ export default function ChildrenPage() {
               onClick={handleAddChild}
               disabled={loading || !familyId}
             >
-              Add Child
+              {t('addChild')}
             </Button>
           </Box>
 
@@ -186,10 +188,10 @@ export default function ChildrenPage() {
                   <CardContent sx={{ textAlign: 'center', py: 8 }}>
                     <ChildCare sx={{ fontSize: 64, color: 'text.secondary', mb: 2 }} />
                     <Typography variant="h6" component="h2" color="text.secondary" gutterBottom>
-                      No children added yet
+                      {t('noChildren')}
                     </Typography>
                     <Typography variant="body2" color="text.secondary" sx={{ mb: 3 }}>
-                      Add your first child to start tracking their activities
+                      {t('noChildrenSubtitle')}
                     </Typography>
                     <Button
                       variant="contained"
@@ -197,7 +199,7 @@ export default function ChildrenPage() {
                       onClick={handleAddChild}
                       disabled={!familyId}
                     >
-                      Add First Child
+                      {t('addFirstChild')}
                     </Button>
                   </CardContent>
                 </Card>
@@ -257,7 +259,7 @@ export default function ChildrenPage() {
                           fontWeight="600"
                           sx={{ mt: 1 }}
                         >
-                          Age: {calculateAge(child.birthDate)}
+                          {t('age')}: {calculateAge(child.birthDate)}
                         </Typography>
                       </CardContent>