Fix downloads never finishing due to duplicate AppEnvironment

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().

Yattee/YatteeApp.swift

@@ -27,7 +27,7 @@ struct YatteeApp: App {
     @NSApplicationDelegateAdaptor(AppDelegate.self) private var appDelegate
     #endif
 
-    @State private var appEnvironment = AppEnvironment()
+    @State private var appEnvironment = AppEnvironment.shared
     @State private var backgroundTasksRegistered = false
     @State private var showingClipboardAlert = false
     @State private var detectedClipboardURL: URL?