simplify: dedupe boilerplate and remove dead code across all layers

lib/model/account_data.dart

@@ -196,15 +196,10 @@ class AccountData {
   /// Prefer this over embedding credentials in URLs — error logs and crash
   /// reports often capture the URL but not headers.
   String getBasicAuthHeader() {
-    if (!isPopulated()) {
-      throw Exception(
-        'AccountData (e.g. username or password) is not initialized!',
-      );
-    }
+    _requirePopulated();
     // Prefer the scoped app password once available; it survives real-password
     // rotation and is what the push-v2 registration is bound to.
-    final secret = _appPassword ?? _password;
-    return 'Basic ${base64Encode(utf8.encode('$_username:$secret'))}';
+    return _basicAuth(_appPassword ?? _password!);
   }
 
   /// Basic-auth header using the Talk app password — authenticates the
@@ -212,29 +207,32 @@ class AccountData {
   /// talk password has not been minted yet; callers treat that as a failed
   /// talk registration and retry on the next start.
   String getTalkBasicAuthHeader() {
-    if (!isPopulated()) {
-      throw Exception(
-        'AccountData (e.g. username or password) is not initialized!',
-      );
-    }
+    _requirePopulated();
     if (!hasAppPasswordTalk()) {
       throw StateError('Talk app password not available yet');
     }
-    return 'Basic ${base64Encode(utf8.encode('$_username:$_appPasswordTalk'))}';
+    return _basicAuth(_appPasswordTalk!);
   }
 
   /// Basic-auth header that always uses the real password. Needed exactly once,
   /// to mint the app password via `core/getapppassword` (an app password cannot
   /// mint another).
   String getRealPasswordBasicAuthHeader() {
+    _requirePopulated();
+    return _basicAuth(_password!);
+  }
+
+  void _requirePopulated() {
     if (!isPopulated()) {
       throw Exception(
         'AccountData (e.g. username or password) is not initialized!',
       );
     }
-    return 'Basic ${base64Encode(utf8.encode('$_username:$_password'))}';
   }
 
+  String _basicAuth(String secret) =>
+      'Basic ${base64Encode(utf8.encode('$_username:$secret'))}';
+
   /// Convenience wrapper around [getBasicAuthHeader] returning a single-entry
   /// header map ready to merge into HTTP request headers.
   Map<String, String> authHeaders() => {'Authorization': getBasicAuthHeader()};

lib/model/endpoint_data.dart

@@ -29,13 +29,10 @@ class EndpointData {
 
   EndpointData._construct();
 
-  EndpointMode getEndpointMode() {
-    late String existingName;
-    existingName = AccountData().getUsername();
-    return existingName.startsWith('google')
-        ? EndpointMode.stage
-        : EndpointMode.live;
-  }
+  EndpointMode getEndpointMode() =>
+      AccountData().getUsername().startsWith('google')
+      ? EndpointMode.stage
+      : EndpointMode.live;
 
   Endpoint nextcloud() => EndpointOptions(
     live: Endpoint(domain: 'cloud.marianum-fulda.de'),