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().
The background URLSession could be in an invalid state when downloadTask(with:)
is called, because invalidateAndCancel() is asynchronous internally. This adds
an ObjC exception handler to catch NSExceptions from CFNetwork, nil guards on
the session, and safer session lifecycle management (nil after invalidation,
finishTasksAndInvalidate for cellular toggle).
