feat: add optional subtitle style editor with live preview

examples/App.css

@@ -5,8 +5,9 @@
 }
 
 body {
-  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen', 'Ubuntu', 'Cantarell', 'Fira Sans',
-    'Droid Sans', 'Helvetica Neue', sans-serif;
+  font-family:
+    -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen', 'Ubuntu', 'Cantarell',
+    'Fira Sans', 'Droid Sans', 'Helvetica Neue', sans-serif;
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
   background-color: #0f172a;
@@ -70,6 +71,9 @@ body {
 .controls-section {
   max-width: 1200px;
   margin: 0 auto;
+  display: flex;
+  flex-direction: column;
+  gap: 0.85rem;
 }
 
 .url-input {
@@ -122,6 +126,20 @@ body {
   color: white;
 }
 
+.toggle-row {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.6rem;
+  color: #cbd5e1;
+  font-size: 0.95rem;
+}
+
+.toggle-row input[type='checkbox'] {
+  width: 16px;
+  height: 16px;
+  accent-color: #ef4444;
+}
+
 .features-section {
   margin-top: 4rem;
 }