Closes WOOMOB-1830
Also closes WOOMOB-1836

Description

When migrating to UIScene lifecycle in #16230, we moved AppCoordinator from AppDelegate to UISceneDelegate. Since the authenticator is initialized as part of AppCoordinator, the app can crash if a user launches the app while authenticated with site credentials. The crash chain is below:

1. AppDelegate.willFinishLaunchingWithOptions (line 53) starts
2. Line 60: Calls stores.initializeAfterDependenciesAreInitialized()
   → This calls restoreSessionSiteIfPossible() (DefaultStoresManager.swift:179)
   → Which calls restoreWordPressSite(with: url) (line 747)
   → Which calls WordPressAuthenticator.fetchSiteInfo (line 782)
   → 💥 CRASH: WordPressAuthenticator.shared wasn't initialized yet!
3. Much later, SceneDelegate.scene(_:willConnectTo:options:) is called
4. Line 20: AppCoordinator is created
   • In AppCoordinator.init(), line 67 calls configureAuthenticator()
   • Which calls authenticationManager.initialize() at line 241
   • But it's too late - the crash already happened!

The solution is to move the initialization of the coordinator from AppCoordinator's configureAuthenticator to AppDelegate before initializeAfterDependenciesAreInitialized is triggered.

Test Steps

The crash is a timing issue - it requires that WordPressSiteAction.fetchSiteInfo returns almost immediately, triggering WordPressAuthenticator.fetchSiteInfo before SceneDelegate's done setting up AppCoordinator. So it's not straightforward to reproduce the crash.

The best bet is to use the debugger: add a breakpoint in the first line of DefaultStoresManager.restoreWordPressSite then do the following steps:

1. Switch to the code before this fix.
2. Log in to a self-hosted store with site credentials.
3. Run the debugger with the above breakpoint.
4. When the breakpoint pauses, check p WordPressAuthenticator.shared. You should see in Xcode console: error: Execution was interrupted, reason: EXC_BREAKPOINT (code=1, subcode=0x197309f0c). The process has been returned to the state before expression evaluation.
5. Switch to the fix in this PR and repeat the steps 1-4. Confirm that you see details about the authenticator instead of the above error.

Screenshots

N/A

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.