replaced account data singleton with a session model

lib/api/demo/data/demo_talk.dart

@@ -1,4 +1,4 @@
-import '../../../model/account_data.dart';
+import '../../../session/session_manager.dart';
 import '../../marianumcloud/talk/chat/get_chat_response.dart';
 import '../../marianumcloud/talk/room/get_room_response.dart';
 import '../demo_persona.dart';
@@ -204,7 +204,8 @@ class DemoTalk {
         id: base + 2,
         token: token,
         ago: const Duration(days: 1, hours: 6),
-        message: 'Danke! Können wir Aufgabe 5 nächste Stunde nochmal besprechen?',
+        message:
+            'Danke! Können wir Aufgabe 5 nächste Stunde nochmal besprechen?',
       ),
       _msg(
         id: base + 3,
@@ -366,7 +367,7 @@ class DemoTalk {
   }) => _msg(
     id: id,
     token: token,
-    actor: AccountData().getUsername(),
+    actor: SessionManager().requireNextcloud().username,
     display: DemoPersona.studentName,
     ago: ago,
     message: message,

lib/api/demo/demo_mode.dart

@@ -1,4 +1,4 @@
-import '../../model/account_data.dart';
+import '../../session/session_manager.dart';
 
 /// Central switch for the client-side demo mode.
 ///
@@ -8,7 +8,7 @@ import '../../model/account_data.dart';
 /// Play reviewers and automated screenshot runs see a fully populated app
 /// without a real account and without any network dependency.
 ///
-/// The flag is persisted through [AccountData.isDemo], so a demo session
+/// The flag is persisted through [Session.isDemo], so a demo session
 /// survives cold starts exactly like a normal login. It works in release builds
 /// too — reviewers run the shipped release build.
 class DemoMode {
@@ -23,5 +23,5 @@ class DemoMode {
       username.trim().toLowerCase().startsWith(usernamePrefix);
 
   /// True while the active session is a demo session.
-  static bool get active => AccountData().isDemo;
+  static bool get active => SessionManager().isDemo;
 }