Add save progress UI and fix MMS errors

Show a progress bar during MP3 generation, update styles, and improve MMS error messaging.

client/static/app.js

@@ -5,10 +5,47 @@ const editBtn = document.getElementById("edit-btn");
 const deleteBtn = document.getElementById("delete-btn");
 const cancelBtn = document.getElementById("cancel-btn");
 const downloadLink = document.getElementById("download-link");
+const progressWrap = document.getElementById("save-progress");
+const progressBar = document.getElementById("save-progress-bar");
 
 let items = [];
 let editMode = false;
 const selectedIds = new Set();
+let progressTimer = null;
+
+function startProgress() {
+  let value = 0;
+  progressWrap.classList.remove("hidden");
+  progressBar.style.width = "0%";
+  progressBar.setAttribute("aria-valuenow", "0");
+
+  if (progressTimer) {
+    clearInterval(progressTimer);
+  }
+
+  progressTimer = setInterval(() => {
+    value = Math.min(value + Math.random() * 8 + 2, 90);
+    progressBar.style.width = `${value}%`;
+    progressBar.setAttribute("aria-valuenow", `${Math.round(value)}`);
+  }, 300);
+}
+
+function finishProgress(success = true) {
+  if (progressTimer) {
+    clearInterval(progressTimer);
+    progressTimer = null;
+  }
+
+  progressBar.style.width = "100%";
+  progressBar.setAttribute("aria-valuenow", "100");
+
+  const delay = success ? 400 : 1200;
+  setTimeout(() => {
+    progressBar.style.width = "0%";
+    progressBar.setAttribute("aria-valuenow", "0");
+    progressWrap.classList.add("hidden");
+  }, delay);
+}
 
 function setEditMode(isEdit) {
   editMode = isEdit;
@@ -69,6 +106,7 @@ async function handleSave() {
     return;
   }
 
+  startProgress();
   const res = await fetch("/api/tts", {
     method: "POST",
     headers: { "Content-Type": "application/json" },
@@ -78,12 +116,14 @@ async function handleSave() {
   if (!res.ok) {
     const err = await res.json().catch(() => ({}));
     alert(err.detail || "저장에 실패했습니다.");
+    finishProgress(false);
     return;
   }
 
   const created = await res.json();
   items.unshift(created);
   renderList();
+  finishProgress(true);
 }
 
 async function handleItemClick(item) {