Fix: Verhindere Ueberschreiben von Nextcloud-Daten mit leeren Arrays

- In saveNextcloudConfig(): Nur speichern, wenn Daten vorhanden sind
- Falls keine lokalen Daten, aber Nextcloud hat Daten: behalte Nextcloud-Daten
- Verhindert Datenverlust, wenn Nextcloud bereits Daten hat

Fixes: Datenverlust bei Nextcloud-Anmeldung

Co-authored-by: trevor1969 <trevor1969@users.noreply.github.com>

app.js

@@ -993,8 +993,34 @@ class TeeTracker {
                 // This will automatically fall back to localStorage if Nextcloud has no data
                 await this.loadData();
                 
-                // Save the loaded/merged data to Nextcloud
+                // Only save to Nextcloud if we actually have data to save
+                // This prevents overwriting existing Nextcloud data with empty arrays
+                if (this.teas.length > 0 || this.entries.length > 0) {
                     await this.saveData();
+                } else {
+                    // No data to save, but check if Nextcloud has existing data
+                    const existingTeas = await ncStorage.loadFile(DATA_DIRECTORY + 'teas.json');
+                    const existingEntries = await ncStorage.loadFile(DATA_DIRECTORY + 'entries.json');
+                    
+                    // If Nextcloud has data, keep it and load it
+                    if (existingTeas || existingEntries) {
+                        if (existingTeas) {
+                            this.teas = existingTeas.map(tea => ({
+                                ...tea,
+                                organic: tea.organic !== undefined ? tea.organic : false,
+                                rating: tea.rating !== undefined ? tea.rating : 3
+                            }));
+                        }
+                        if (existingEntries) {
+                            this.entries = existingEntries.map(entry => ({
+                                ...entry,
+                                teaspoons: entry.teaspoons !== undefined ? entry.teaspoons : 1
+                            }));
+                        }
+                        // Don't save, just use the existing Nextcloud data
+                    }
+                    // If Nextcloud has no data and we have no data, nothing to do
+                }
                 
                 this.showStatusMessage('✅ Konfiguration gespeichert und Daten synchronisiert!', 'success');
             } else {