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>

styles.css

@@ -936,3 +936,38 @@ header p {
     outline: none;
     border-color: var(--primary-color);
 }
+
+/* Background Image Settings */
+.bg-preview {
+    margin-top: 10px;
+}
+
+.bg-preview img {
+    max-width: 200px;
+    max-height: 150px;
+    border-radius: var(--border-radius);
+    box-shadow: var(--box-shadow);
+    object-fit: cover;
+}
+
+/* Background Image for App */
+.app-container {
+    background-image: var(--bg-image, none);
+    background-size: cover;
+    background-position: center;
+    background-attachment: fixed;
+    background-repeat: no-repeat;
+}
+
+/* Overlay for better text readability */
+.app-container::before {
+    content: '';
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    background-color: rgba(255, 255, 255, 0.85);
+    z-index: -1;
+    pointer-events: none;
+}