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/exceptions/HttpError.js

@@ -1,10 +1,54 @@
+/**
+ * HttpError mit Unterstützung für Fehlercodes
+ * 
+ * Verwendung:
+ * - new HttpError('Fehlermeldung', 400) - Legacy, wird weiterhin unterstützt
+ * - new HttpError({ code: 'ERROR_USER_NOT_FOUND' }, 404) - Mit Fehlercode
+ * - new HttpError({ code: 'ERROR_MEMBER_NOT_FOUND', params: { memberId: 123 } }, 404) - Mit Parametern
+ */
 class HttpError extends Error {
-    constructor(message, statusCode) {
-        super(message);
+    constructor(messageOrError, statusCode) {
+        // Unterstützung für beide Formate:
+        // 1. Legacy: new HttpError('Fehlermeldung', 400)
+        // 2. Neu: new HttpError({ code: 'ERROR_CODE', params: {...} }, 400)
+        if (typeof messageOrError === 'string') {
+            // Legacy-Format
+            super(messageOrError);
+            this.errorCode = null;
+            this.errorParams = null;
+        } else if (messageOrError && typeof messageOrError === 'object' && messageOrError.code) {
+            // Neues Format mit Fehlercode
+            super(messageOrError.code); // Für Stack-Trace
+            this.errorCode = messageOrError.code;
+            this.errorParams = messageOrError.params || null;
+        } else {
+            // Fallback
+            super('Unknown error');
+            this.errorCode = null;
+            this.errorParams = null;
+        }
+        
         this.name = this.constructor.name;
-        this.statusCode = statusCode; 
+        this.statusCode = statusCode || 500;
         Error.captureStackTrace(this, this.constructor);
     }
+    
+    /**
+     * Gibt das Fehler-Objekt für die API-Antwort zurück
+     * @returns {object} Fehler-Objekt mit code und optional params
+     */
+    toJSON() {
+        if (this.errorCode) {
+            return {
+                code: this.errorCode,
+                ...(this.errorParams && { params: this.errorParams })
+            };
+        }
+        // Legacy: Gib die Nachricht zurück
+        return {
+            message: this.message
+        };
+    }
 }
 
 export default HttpError;