feat: add onboarding tutorial for new users

- Introduced a modular tutorial layer to guide new parents through the app setup process.
- Implemented a 3-step forced intro after first sign-in (PIN setup → child creation → chore creation).
- Added just-in-time contextual hints for various features as users encounter them.
- Persisted user progress on the backend with new fields in the User model.
- Created a new tutorial controller and step registry in the frontend for managing tutorial states.
- Added Help button for easy access to tutorial tips and a restart option in the user profile.
- Ensured accessibility and mobile responsiveness for the tutorial overlay.
- Included tests for backend and frontend functionalities related to the tutorial.

frontend/src/App.vue

@@ -3,12 +3,14 @@
   <router-view />
   <TutorialOverlay />
   <TutorialChildModeOffer />
+  <HelpButton />
 </template>
 
 <script setup lang="ts">
 import BackendEventsListener from '@/components/BackendEventsListener.vue'
 import TutorialOverlay from '@/tutorial/TutorialOverlay.vue'
 import TutorialChildModeOffer from '@/tutorial/TutorialChildModeOffer.vue'
+import HelpButton from '@/tutorial/HelpButton.vue'
 import { checkAuth } from '@/stores/auth'
 
 checkAuth()