SwiftUI evaluates a @State default-value autoclosure more than once,
keeping only the first instance but still running the side effects of
the discarded ones. Each AppEnvironment() built its own DownloadManager,
and each DownloadManager registered a background URLSession under the
same identifier. The download task started on the surviving instance's
session, but iOS delivered the completion delegate callback to the other
(leaked) instance, whose activeDownloads was empty - so the finished
file was dropped and the progress spinner spun forever at 0 KB.
- Make AppEnvironment a process-wide singleton (static let shared) and
reference it from the App's @State, guaranteeing exactly one instance
(and one background session, one DataManager/CloudKit stack).
- Make DownloadManager.setDownloadSettings idempotent: only create the
background session when none exists, so a re-entrant call never
invalidateAndCancels the live session and kills in-flight downloads.
Cellular changes already route through refreshCellularAccessSetting().
Delete the multi-page onboarding sheet. On first launch the app now
silently imports any v1 instances from UserDefaults (splitting embedded
basic-auth credentials out of the URL and into the Keychain) and then,
if the device is signed in to iCloud, shows a single alert offering to
enable sync. Accepting shows a blocking progress overlay until the
initial upload completes.
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.
