Improve delete UX and TTS audio quality

Clear text on delete and enhance TTS output with Korean voice selection and ffmpeg audio normalization.

client/static/app.js

@@ -131,6 +131,9 @@ async function handleDelete() {
   const data = await res.json();
   const deletedSet = new Set(data.deleted || []);
   items = items.filter((item) => !deletedSet.has(item.id));
+  textInput.value = "";
+  downloadLink.href = "#";
+  downloadLink.classList.add("hidden");
   setEditMode(false);
 }