refactor: 移除 relay 服务中的 model 参数，Dify /chat-messages 不需要此字段

test.html

@@ -194,9 +194,6 @@
     .success-text { color: #86efac; }
     .warn-text { color: #fcd34d; }
 
-    /* Model select */
-    #model { margin-bottom: 20px; }
-
     /* Streaming indicator */
     .streaming-indicator {
       display: none;
@@ -256,10 +253,6 @@
         <label>Dify API 密钥</label>
         <input type="password" id="apiKey" placeholder="app-xxxxxxxx" />
       </div>
-      <div class="field">
-        <label>模型名称</label>
-        <input type="text" id="modelName" placeholder="dify" />
-      </div>
     </div>
 
     <div class="btn-row">
@@ -286,7 +279,6 @@
   // ── Helpers ────────────────────────────────────────────────
   const relayUrl = () => document.getElementById('relayUrl').value.replace(/\/$/, '');
   const apiKey   = () => document.getElementById('apiKey').value.trim();
-  const model    = () => document.getElementById('modelName').value.trim() || 'dify';
 
   function setStatus(type, text) {
     const dot  = document.getElementById('statusDot');
@@ -354,7 +346,6 @@
     setStatus('idle', '请求中…');
 
     const payload = {
-      model: model(),
       stream: false,
       messages: [
         { role: 'user', content: '你好，请用一句话介绍自己。' }