feat: Implement GraphQL mutations for activities and children

Add complete GraphQL mutation support for activity tracking and child management:

**Activity Mutations:**
- createActivity: Create new activities (feeding, sleep, diaper, medication)
- updateActivity: Update existing activities
- deleteActivity: Delete activities

**Child Mutations:**
- createChild: Add new children to families
- updateChild: Update child information
- deleteChild: Soft delete children

**Implementation Details:**
- Created GraphQL input types (CreateActivityInput, UpdateActivityInput, CreateChildInput, UpdateChildInput)
- Implemented ActivityResolver with full CRUD mutations
- Implemented ChildResolver with full CRUD mutations
- Registered resolvers in GraphQL module with TrackingService and ChildrenService
- Auto-generated GraphQL schema with all mutations
- All mutations protected with GqlAuthGuard for authentication
- Support for JSON metadata fields and Gender enum

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

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

maternal-web/package.json

@@ -15,6 +15,7 @@
     "test:e2e:headed": "playwright test --headed"
   },
   "dependencies": {
+    "@apollo/client": "^4.0.7",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.1",
     "@hookform/resolvers": "^5.2.2",
@@ -28,6 +29,7 @@
     "date-fns": "^4.1.0",
     "focus-trap-react": "^11.0.4",
     "framer-motion": "^12.23.22",
+    "graphql": "^16.11.0",
     "next": "^15.5.4",
     "next-pwa": "^5.6.0",
     "react": "^19.2.0",