fix: lower copy code button z-index to stay below settings panel

- Reduce z-index from 10 to 5 for copy code buttons
- Prevents buttons from appearing on top of settings panel
- Settings panel has z-index 10 and should stay on top when open
- Applied to both builtin and PicoCSS styles

src/uncloseai-modal-builtin.css

@@ -811,7 +811,7 @@ dialog#tts-modal {
   cursor: pointer;
   opacity: 0.8;
   transition: opacity 0.2s;
-  z-index: 10;
+  z-index: 5;
 }
 
 .uncloseai-code-copy-btn:hover {

src/uncloseai-modal-pico.css

@@ -867,7 +867,7 @@ dialog#tts-modal::backdrop {
   cursor: pointer;
   opacity: 0.8;
   transition: opacity 0.2s;
-  z-index: 10;
+  z-index: 5;
 }
 
 .uncloseai-code-copy-btn:hover {