replaced account data singleton with a session model

lib/widget/file_viewer/unknown_preview_block.dart

@@ -1,8 +1,8 @@
 import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 
-import '../../model/account_data.dart';
 import '../../model/endpoint_data.dart';
+import '../../session/session_manager.dart';
 import '../../share_intent/remote_file_ref.dart';
 import '../app_progress_indicator.dart';
 
@@ -67,7 +67,7 @@ class _UnknownPreviewBlockState extends State<UnknownPreviewBlock> {
       width: _previewSize,
       height: _previewSize,
       child: CachedNetworkImage(
-        httpHeaders: AccountData().authHeaders(),
+        httpHeaders: SessionManager().requireNextcloud().authHeaders,
         imageUrl: _ncPreviewUrl(remote, width: 360),
         fadeInDuration: Duration.zero,
         fadeOutDuration: Duration.zero,

lib/widget/user_avatar.dart

@@ -8,9 +8,9 @@ import 'package:flutter_svg/flutter_svg.dart';
 import 'package:http/http.dart' as http;
 
 import '../api/http_errors.dart';
-import '../model/account_data.dart';
 import '../model/endpoint_data.dart';
 import '../push/push_avatar.dart';
+import '../session/session_manager.dart';
 import 'a11y/a11y_labels.dart';
 import 'avatar_disk_cache.dart';
 
@@ -162,7 +162,9 @@ Future<AvatarPayload?> _fetchAvatarPayload(String url) async {
     () => http.get(
       Uri.parse(url),
       headers: {
-        ...AccountData().authHeaders(),
+        // User avatars are public in Nextcloud, so they also load while the
+        // active session has no Nextcloud identity.
+        ...?SessionManager().current?.nextcloud?.authHeaders,
         'Accept': 'image/png,image/jpeg,image/webp,image/svg+xml',
       },
     ),
@@ -368,7 +370,6 @@ class _UserAvatarState extends State<UserAvatar> {
     return listEquals(a.bytes, b.bytes);
   }
 
-
   @override
   Widget build(BuildContext context) {
     final radius = widget.size.toDouble();