Refactor error handling and localization in frontend components

This commit enhances the error handling and user interface of various frontend components by integrating localization support. It updates error messages and titles across multiple views and dialogs to utilize the translation function, ensuring a consistent user experience in different languages. Additionally, it refines the handling of error messages in the MyTischtennis account and member transfer settings, improving clarity and user feedback during operations.

backend/server.js

@@ -52,6 +52,7 @@ import trainingGroupRoutes from './routes/trainingGroupRoutes.js';
 import trainingTimeRoutes from './routes/trainingTimeRoutes.js';
 import schedulerService from './services/schedulerService.js';
 import { requestLoggingMiddleware } from './middleware/requestLoggingMiddleware.js';
+import HttpError from './exceptions/HttpError.js';
  
 const app = express();
 const port = process.env.PORT || 3005;
@@ -86,12 +87,25 @@ app.use((err, req, res, next) => {
   }
 
   const status = err?.statusCode || err?.status || 500;
-  const message = err?.message || 'Interner Serverfehler';
+  
+  // Unterstützung für Fehlercodes
+  let errorResponse;
+  if (err instanceof HttpError && err.errorCode) {
+    // Neues Format mit Fehlercode
+    errorResponse = err.toJSON();
+  } else {
+    // Legacy-Format: String-Nachricht
+    const message = err?.message || 'Interner Serverfehler';
+    errorResponse = {
+      message
+    };
+  }
 
   const response = {
     success: false,
-    message,
-    error: message
+    ...errorResponse,
+    // Für Rückwärtskompatibilität: error-Feld mit Nachricht
+    error: errorResponse.message || errorResponse.code || 'Interner Serverfehler'
   };
 
   if (process.env.NODE_ENV === 'dev' || process.env.NODE_ENV === 'development') {