fix: 搜索无结果时新建按钮改为el-select的empty插槽，避免被遮挡

- 将「新建客户」按钮从 el-select 下方的独立 div 移入 #empty 插槽
- 搜索无匹配时，下拉面板直接显示「未找到xxx」+ 新建按钮
- 不再被原生「无数据」空状态遮挡
- 移动端+桌面端五个页面统一修复

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/views/mobile/VisitForm.vue

@@ -235,16 +235,19 @@ async function handleDelete() {
           style="width:100%"
         >
           <el-option v-for="c in customers" :key="c.id" :label="c.name" :value="c.id" />
+          <template #empty>
+            <div v-if="customerSearch" class="select-empty-create">
+              <p style="color:var(--warm-gray);font-size:13px;margin:0 0 8px">未找到「{{ customerSearch }}」</p>
+              <button type="button" class="quick-create-btn" @click.stop="handleQuickCreate">
+                <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+                  <line x1="12" y1="5" x2="12" y2="19"></line>
+                  <line x1="5" y1="12" x2="19" y2="12"></line>
+                </svg>
+                新建客户「{{ customerSearch }}」
+              </button>
+            </div>
+          </template>
         </el-select>
-        <div v-if="customerSearch && !customers.some(c => c.name === customerSearch)" style="margin-top:8px">
-          <button type="button" class="quick-create-btn" @click="handleQuickCreate">
-            <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-              <line x1="12" y1="5" x2="12" y2="19"></line>
-              <line x1="5" y1="12" x2="19" y2="12"></line>
-            </svg>
-            新建客户「{{ customerSearch }}」
-          </button>
-        </div>
       </el-form-item>
 
       <el-form-item>
@@ -494,6 +497,7 @@ async function handleDelete() {
   color: var(--vermilion);
   background: rgba(184,71,46,0.03);
 }
+.select-empty-create { padding: 8px 12px; text-align: center; }
 
 /* ═══ Photo Area ═══ */
 .photo-area {