iOS Share Extension: pick the destination inside the share sheet

SwiftUI folder picker in the extension (WebDAV PROPFIND), account shared via
a Keychain group, upload on a background session that outlives the extension
and is finished by the app. The inbox + notification hand-off remains as the
fallback (no account / 'choose later'). Shared code moves to ios/Shared.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ios/Runner/AppDelegate.swift

@@ -30,7 +30,7 @@ import UserNotifications
     handleEventsForBackgroundURLSession identifier: String,
     completionHandler: @escaping () -> Void
   ) {
-    if identifier == TransferManager.sessionIdentifier {
+    if identifier == TransferManager.sessionIdentifier || identifier == ShareUpload.sessionIdentifier {
       TransferManager.shared.backgroundCompletionHandler = completionHandler
     } else {
       super.application(