feat: add live preview of reading font in settings

- Add preview section showing Lorem Ipsum passage
- Preview updates instantly when font is changed
- Load font dynamically for preview
- Style preview to match reader appearance
- Helps users see font changes before saving

src/index.css

@@ -1487,6 +1487,39 @@ body {
   gap: 0.5rem;
 }
 
+.setting-preview {
+  margin: 1.5rem 0;
+  padding: 1rem;
+  background: #1a1a1a;
+  border: 1px solid #333;
+  border-radius: 8px;
+}
+
+.preview-label {
+  font-size: 0.875rem;
+  color: #999;
+  margin-bottom: 0.75rem;
+  font-weight: 500;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+.preview-content {
+  color: #ddd;
+  line-height: 1.7;
+}
+
+.preview-content h3 {
+  margin: 0 0 1rem 0;
+  font-size: 1.5rem;
+  color: #fff;
+}
+
+.preview-content p {
+  margin: 0.75rem 0;
+  font-size: 1rem;
+}
+
 .setting-select {
   width: 100%;
   padding: 0.5rem;