implemented a backend-independent emergency notice system

lib/storage/dev_tools_settings.dart

@@ -17,12 +17,18 @@ class DevToolsSettings {
   @JsonKey(defaultValue: '')
   String marianumConnectCustomUrl;
 
+  /// Optional override for the backend-independent emergency-notice source.
+  /// Empty falls back to [emergencyNoticeDefaultUrl].
+  @JsonKey(defaultValue: '')
+  String emergencyNoticeUrl;
+
   DevToolsSettings({
     required this.showPerformanceOverlay,
     required this.checkerboardOffscreenLayers,
     required this.checkerboardRasterCacheImages,
     this.marianumConnectEndpoint = MarianumConnectEndpoint.live,
     this.marianumConnectCustomUrl = '',
+    this.emergencyNoticeUrl = '',
   });
 
   // Resolves the effective base URL, falling back to live when the custom URL
@@ -43,6 +49,14 @@ class DevToolsSettings {
   static const String liveUrl = 'https://connect.marianum-fulda.de';
   static const String betaUrl = 'https://connect-beta.marianum-fulda.de';
 
+  /// Compiled-in source for the backend-independent emergency notice. Must live
+  /// on infrastructure that is reachable even when MarianumConnect is down.
+  static const String emergencyNoticeDefaultUrl = 'https://www.marianum-fulda.de/~aushang/marMobile.override';
+
+  String? resolveEmergencyNoticeUrl() =>
+      sanitizeCustomUrl(emergencyNoticeUrl) ??
+      sanitizeCustomUrl(emergencyNoticeDefaultUrl);
+
   /// `true` in builds where plaintext HTTP custom endpoints are still allowed
   /// (debug, profile). Release builds keep this `false` and the picker
   /// rejects `http://` entirely.