fix(myTischtennis): add detailed diagnostics for login failures

- Introduced failure diagnostics logging during the login process to capture URL, cookie names, and body preview on errors.
- Enhanced error handling to provide clearer insights into login issues, particularly related to CAPTCHA and password failures.
- Improved console warnings for better visibility into authentication problems encountered during login attempts.

backend/clients/myTischtennisClient.js

@@ -585,6 +585,7 @@ class MyTischtennisClient {
       }
       if (!authCookieObj || !authCookieObj.value) {
         let errorText = null;
+        let failureDiagnostics = null;
         try {
           const textContent = await page.locator('body').innerText({ timeout: 1000 });
           if (textContent?.includes('Captcha-Bestätigung fehlgeschlagen')) {
@@ -593,12 +594,24 @@ class MyTischtennisClient {
           if (!errorText && textContent?.includes('Passwort')) {
             errorText = 'Login vermutlich fehlgeschlagen (Passwort oder CAPTCHA)';
           }
+
+          const currentUrl = page.url();
+          const allCookies = await context.cookies();
+          const cookieNames = allCookies.map((c) => c.name);
+          failureDiagnostics = {
+            url: currentUrl,
+            cookieNames,
+            bodyPreview: String(textContent || '').slice(0, 320)
+          };
         } catch (_e) {
           // ignore text read errors
         }
         if (!errorText && detectedSubmitError) {
           errorText = detectedSubmitError;
         }
+        if (failureDiagnostics) {
+          console.warn('[myTischtennisClient.playwright] Login failure diagnostics:', failureDiagnostics);
+        }
         return {
           success: false,
           error: errorText