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

@@ -135,7 +135,8 @@ class NextcloudStorage {
 
             if (response.ok) {
                 const text = await response.text();
-                return text ? JSON.parse(text) : null;
+                // Return empty array if file is empty, otherwise parse JSON
+                return text.trim() === '' ? [] : (text ? JSON.parse(text) : null);
             } else if (response.status === 404) {
                 return null;
             }
@@ -280,12 +281,31 @@ class TeeTracker {
             const teas = await this.nextcloudStorage.loadFile(DATA_DIRECTORY + 'teas.json');
             const entries = await this.nextcloudStorage.loadFile(DATA_DIRECTORY + 'entries.json');
             
+            // Always load from localStorage first as backup
+            const localTeas = localStorage.getItem(STORAGE_KEY_TEAS);
+            const localEntries = localStorage.getItem(STORAGE_KEY_ENTRIES);
+            
+            // Load from Nextcloud if available, otherwise use local data
             if (teas) {
                 this.teas = teas.map(tea => ({
                     ...tea,
                     organic: tea.organic !== undefined ? tea.organic : false,
                     rating: tea.rating !== undefined ? tea.rating : 3
                 }));
+            } else if (localTeas) {
+                // Nextcloud has no teas, use local data
+                try {
+                    this.teas = JSON.parse(localTeas).map(tea => ({
+                        ...tea,
+                        organic: tea.organic !== undefined ? tea.organic : false,
+                        rating: tea.rating !== undefined ? tea.rating : 3
+                    }));
+                } catch (error) {
+                    console.error('Error loading local teas:', error);
+                    this.teas = [];
+                }
+            } else {
+                this.teas = [];
             }
             
             if (entries) {
@@ -293,11 +313,19 @@ class TeeTracker {
                     ...entry,
                     teaspoons: entry.teaspoons !== undefined ? entry.teaspoons : 1
                 }));
-            }
-            
-            // If Nextcloud loading failed, fall back to localStorage
-            if (!teas || !entries) {
-                this.loadFromLocalStorage();
+            } else if (localEntries) {
+                // Nextcloud has no entries, use local data
+                try {
+                    this.entries = JSON.parse(localEntries).map(entry => ({
+                        ...entry,
+                        teaspoons: entry.teaspoons !== undefined ? entry.teaspoons : 1
+                    }));
+                } catch (error) {
+                    console.error('Error loading local entries:', error);
+                    this.entries = [];
+                }
+            } else {
+                this.entries = [];
             }
         } else {
             // Load from localStorage
@@ -949,30 +977,6 @@ class TeeTracker {
                     return;
                 }
                 
-                // Load existing data from localStorage before switching to Nextcloud
-                // This prevents data loss when connecting to Nextcloud for the first time
-                const existingTeas = localStorage.getItem(STORAGE_KEY_TEAS);
-                const existingEntries = localStorage.getItem(STORAGE_KEY_ENTRIES);
-                
-                if (existingTeas) {
-                    try {
-                        this.teas = JSON.parse(existingTeas).map(tea => ({
-                            ...tea,
-                            organic: tea.organic !== undefined ? tea.organic : false
-                        }));
-                    } catch (error) {
-                        console.error('Error loading existing teas:', error);
-                    }
-                }
-                
-                if (existingEntries) {
-                    try {
-                        this.entries = JSON.parse(existingEntries);
-                    } catch (error) {
-                        console.error('Error loading existing entries:', error);
-                    }
-                }
-                
                 // Save config WITHOUT password for security
                 // Password will be requested each time or stored in sessionStorage
                 const config = { baseUrl: url, username, path };
@@ -985,8 +989,38 @@ class TeeTracker {
                 this.nextcloudStorage = ncStorage;
                 this.useNextcloud = true;
                 
-                // Save current data to Nextcloud
-                await this.saveData();
+                // Load data from Nextcloud first (this is the critical fix!)
+                // This will automatically fall back to localStorage if Nextcloud has no data
+                await this.loadData();
+                
+                // 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 {