fix: lazy loading, pagination, and optimistic UI (#587, #594, #593)

[Abdullahi-Code9]

Closes #587
Closes #594
Closes #593

Summary

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#587 — PetPhotosScreen: image lazy loading & pagination

• photoService.listPhotos now accepts { page, limit } and passes them as query params instead of fetching all
  photos and slicing client-side
• Grid thumbnails use server-generated 200×200 thumbnails via thumbnailUrl; full-res only loads on tap
• Prefetch triggered when 5 items from end via onEndReachedThreshold

#594 — AppNavigator: lazy loading for non-critical screens

• All non-critical screens already wrapped in React.lazy() + LazyScreen (Suspense + ErrorBoundary)
• Added a top-level Suspense boundary wrapping NavigationContainer with an ActivityIndicator fallback,
  covering the case where a lazy screen suspends before the navigator fully initializes
• Critical screens (Login/Auth, PetList, PetHealthDashboard) remain eagerly loaded

#593 — AppointmentScreen: optimistic UI updates

• Booking is applied to local state immediately on tap
• "confirming…" indicator shown on the card while server confirms
• On success: indicator replaced with "Confirmed ✓"
• On failure: optimistic entry is fully reverted (appointments, confirmingIds, lockedSlots) and an error toast
  with a "Retry" action is shown
• Slot is locked optimistically on tap to prevent double-booking

Testing

• npx tsc --noEmit — no new type errors
• Existing test suite passes (pre-existing infra failures unrelated to these changes)

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