Renames YatteeServerCredentialsManager → BasicAuthCredentialsManager so the
same Keychain-backed username/password storage can be reused for any instance
type that sits behind a reverse proxy requiring HTTP Basic Auth. Adds a
one-time migration that moves existing items from the legacy
'com.yattee.yatteeserver' Keychain service to 'com.yattee.basicauth',
preserving the iCloud-sync attribute. No behavior change for end users.
On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.
