Clear the prompt on 'make another', keep the chosen length

The again-btn handler did input.select() (kept the previous prompt text selected). Clear the textarea instead so each new tape starts from an empty prompt; the length slider is untouched, so the chosen duration carries over.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/ui.js

@@ -521,8 +521,8 @@ export function initUI({
   });
   $("again-btn").addEventListener("click", () => {
     setStage("prompt");
+    input.value = ""; // start fresh; the length slider keeps its last setting
     input.focus();
-    input.select();
   });
   $("modal-close").addEventListener("click", () => onRequestClose());
   window.addEventListener("keydown", (e) => {