Error handling, paging and tests for the contacts app #3849

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

Part of #3844

User Story

As a contacts user, I want the app to handle errors and large address books gracefully so that I can rely on it.

User Value

Today failed requests leave an empty or endless loading view, contact data is logged to the browser console (console.info), large address books are loaded in one request, and the app has no unit tests.

Acceptance Criteria

• Loading errors show an error state with a retry option instead of an empty list
• No contact or address book data is written to the console
• Large address books are loaded in pages (or with lazy loading) instead of a single request
• Unit tests exist for stores, composables and the main views