Add save progress UI and fix MMS errors

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

client/templates/index.html

@@ -12,6 +12,16 @@
         <div class="panel-header">입력 텍스트</div>
         <textarea id="text-input" rows="16" placeholder="텍스트를 입력하세요"></textarea>
         <button id="save-btn" class="primary">mp3 저장</button>
+        <div id="save-progress" class="progress-wrap hidden" aria-label="저장 진행률">
+          <div
+            id="save-progress-bar"
+            class="progress-bar"
+            role="progressbar"
+            aria-valuemin="0"
+            aria-valuemax="100"
+            aria-valuenow="0"
+          ></div>
+        </div>
       </section>
 
       <section class="panel right">