replaced account data singleton with a session model

lib/utils/downloads/download_manager.dart

@@ -6,8 +6,8 @@ import 'package:background_downloader/background_downloader.dart' as bd;
 import 'package:flutter/foundation.dart';
 
 import '../../api/marianumcloud/webdav/webdav_api.dart';
-import '../../model/account_data.dart';
 import '../../notification/notification_service.dart';
+import '../../session/session_manager.dart';
 import '../../share_intent/remote_file_ref.dart';
 import 'download_job.dart';
 
@@ -116,7 +116,7 @@ class DownloadManager {
     final encodedPath = Uri.encodeComponent(remotePath).replaceAll('%2F', '/');
     final task = bd.DownloadTask(
       url: '${WebdavApi.buildWebdavUrl()}$encodedPath',
-      headers: AccountData().authHeaders(),
+      headers: SessionManager().requireNextcloud().authHeaders,
       filename: name,
       baseDirectory: bd.BaseDirectory.temporary,
       directory: _directory,

lib/utils/random_id.dart

@@ -0,0 +1,19 @@
+import 'dart:math';
+
+/// Random hex id from a cryptographic RNG, e.g. for per-install identifiers.
+String randomHexId({int bytes = 16}) {
+  final random = Random.secure();
+  return List<int>.generate(
+    bytes,
+    (_) => random.nextInt(256),
+  ).map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+}
+
+/// Random RFC 4122 version-4 UUID, for ids a server expects in UUID form.
+String randomUuidV4() {
+  final hex = randomHexId();
+  final variant = (int.parse(hex[16], radix: 16) & 0x3 | 0x8).toRadixString(16);
+  return '${hex.substring(0, 8)}-${hex.substring(8, 12)}-'
+      '4${hex.substring(13, 16)}-$variant${hex.substring(17, 20)}-'
+      '${hex.substring(20)}';
+}

lib/utils/session_single_flight.dart

@@ -1,4 +1,4 @@
-import '../model/account_data.dart';
+import '../session/session_manager.dart';
 
 /// Joins concurrent calls into the one already running for the same account
 /// session. A run left over from a signed-out session never blocks the next
@@ -8,7 +8,7 @@ class SessionSingleFlight {
   int? _epoch;
 
   Future<void> run(Future<void> Function() action) {
-    final epoch = AccountData().sessionEpoch;
+    final epoch = SessionManager().sessionEpoch;
     final running = _running;
     if (running != null && _epoch == epoch) return running;
     _epoch = epoch;