Move login flow management to AppCoordinator

TODO:
- [ ] First implement #3618
- [ ] Move login management from `-[MasterTabBarController viewDidAppear]` to `AppCoordinator.start()`
- [ ] In `AppCoordinator.start()` try do not show `SplitViewCoordinator` until login succeed. If possible try to animate transition from authentication to split view (including tab bar)

First resolve issue #3618, then read `MasterTabBarController`'s `viewDidAppear` and `AppCoordinator.start()`. Trace when `SplitViewCoordinator` is shown and how authentication completes. Done means login management starts in `AppCoordinator.start()`, the split view waits for successful login, and the authentication-to-split-view transition includes the tab bar if feasible.

Written by the indexing model from the issue text.