修复钉钉logo显示：使用SVG图标替代外部图片链接

frontend/src/views/Login.vue

@@ -198,13 +198,11 @@
             @click="handleDingTalkLogin"
             :loading="dingTalkLoginLoading"
           >
-            <img 
-              src="https://img.alicdn.com/imgextra/i1/O1CN01CHK9VA1YTzZJxXGf7_!!6000000003022-2-tps-400-400.png" 
-              alt="钉钉" 
-              class="dingtalk-logo"
-              onerror="this.style.display='none'; this.nextSibling.style.display='inline-block';"
-            />
-            <span class="dingtalk-logo-text" style="display: none;">D</span>
+            <!-- 钉钉logo - 使用白色圆形图标，类似钉钉的样式 -->
+            <svg class="dingtalk-logo" viewBox="0 0 24 24" width="20" height="20" fill="none" xmlns="http://www.w3.org/2000/svg">
+              <circle cx="12" cy="12" r="10" stroke="currentColor" stroke-width="2" fill="none"/>
+              <circle cx="12" cy="12" r="4" fill="currentColor"/>
+            </svg>
             钉钉登录
           </a-button>
         </div>