feat: Add background image customization

- Add background image upload in settings
- Add preview for uploaded background images
- Add toggle to enable/disable background
- Store background image in localStorage as base64
- Add CSS for background image display with overlay
- Add currentBgImage property to TeeTracker class
- Add setupBackgroundListeners() method
- Add handleBackgroundUpload(), toggleBackground(), saveBackgroundSettings(), removeBackground(), loadBackgroundSettings() methods

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

app.js

@@ -221,6 +221,7 @@ class TeeTracker {
         this.entries = [];
         this.currentTeaId = null;
         this.charts = {};
+        this.currentBgImage = null;
         
         // Initialize
         this.initStorage();
@@ -664,6 +665,9 @@ class TeeTracker {
             });
         }
         
+        // Background image settings
+        this.setupBackgroundListeners();
+        
         // Nextcloud settings
         this.setupNextcloudListeners();
     }
@@ -756,6 +760,111 @@ class TeeTracker {
         }
     }
     
+    setupBackgroundListeners() {
+        // Background image upload
+        const bgUpload = document.getElementById('bg-image-upload');
+        if (bgUpload) {
+            bgUpload.addEventListener('change', (e) => this.handleBackgroundUpload(e));
+        }
+        
+        // Background enable toggle
+        const bgEnabled = document.getElementById('bg-enabled');
+        if (bgEnabled) {
+            bgEnabled.addEventListener('change', () => this.toggleBackground());
+        }
+        
+        // Save background button
+        const saveBgBtn = document.getElementById('save-bg');
+        if (saveBgBtn) {
+            saveBgBtn.addEventListener('click', () => this.saveBackgroundSettings());
+        }
+        
+        // Remove background button
+        const removeBgBtn = document.getElementById('remove-bg');
+        if (removeBgBtn) {
+            removeBgBtn.addEventListener('click', () => this.removeBackground());
+        }
+        
+        // Load saved background settings
+        this.loadBackgroundSettings();
+    }
+    
+    handleBackgroundUpload(e) {
+        const file = e.target.files[0];
+        if (!file) return;
+        
+        const preview = document.getElementById('bg-preview-img');
+        const previewContainer = document.getElementById('bg-image-preview');
+        const removeBtn = document.getElementById('remove-bg');
+        
+        if (file.type.startsWith('image/')) {
+            const reader = new FileReader();
+            reader.onload = (event) => {
+                preview.src = event.target.result;
+                previewContainer.style.display = 'block';
+                removeBtn.style.display = 'inline-flex';
+                this.currentBgImage = event.target.result;
+            };
+            reader.readAsDataURL(file);
+        }
+    }
+    
+    toggleBackground() {
+        const enabled = document.getElementById('bg-enabled').checked;
+        if (enabled && this.currentBgImage) {
+            document.documentElement.style.setProperty('--bg-image', `url("${this.currentBgImage}")`);
+        } else {
+            document.documentElement.style.setProperty('--bg-image', 'none');
+        }
+    }
+    
+    saveBackgroundSettings() {
+        const enabled = document.getElementById('bg-enabled').checked;
+        const bgImage = this.currentBgImage;
+        
+        if (enabled && bgImage) {
+            localStorage.setItem('teatracker_bg_enabled', 'true');
+            localStorage.setItem('teatracker_bg_image', bgImage);
+            this.showToast('Hintergrundbild gespeichert!', 'success');
+        } else {
+            localStorage.removeItem('teatracker_bg_enabled');
+            localStorage.removeItem('teatracker_bg_image');
+            this.showToast('Hintergrundbild deaktiviert!', 'success');
+        }
+        this.toggleBackground();
+    }
+    
+    removeBackground() {
+        this.currentBgImage = null;
+        document.getElementById('bg-preview-img').src = '';
+        document.getElementById('bg-image-preview').style.display = 'none';
+        document.getElementById('remove-bg').style.display = 'none';
+        document.getElementById('bg-enabled').checked = false;
+        this.toggleBackground();
+    }
+    
+    loadBackgroundSettings() {
+        this.currentBgImage = localStorage.getItem('teatracker_bg_image');
+        const enabled = localStorage.getItem('teatracker_bg_enabled') === 'true';
+        
+        if (this.currentBgImage) {
+            document.getElementById('bg-enabled').checked = enabled;
+            if (enabled) {
+                document.documentElement.style.setProperty('--bg-image', `url("${this.currentBgImage}")`);
+            }
+            
+            // Show preview
+            const preview = document.getElementById('bg-preview-img');
+            const previewContainer = document.getElementById('bg-image-preview');
+            const removeBtn = document.getElementById('remove-bg');
+            if (preview && previewContainer && removeBtn) {
+                preview.src = this.currentBgImage;
+                previewContainer.style.display = 'block';
+                removeBtn.style.display = 'inline-flex';
+            }
+        }
+    }
+    
     // ============================================
     // Nextcloud Functions
     // ============================================