style(settlement): 优化预结算单选择弹窗及支付申请表样式

- 在预结算单选择弹窗中添加el-card包裹form表单，提升视觉层次
- 修改支付申请表结算单选择的placeholder文本为“请输入”
- 使用el-card包裹支付申请表中的参考搜索区块，增强样式一致性
- 调整表单验证提示文本，将“请选择结算单”改为“请输入”

src/views/settlement/components/formal-settlement-editor.vue

@@ -691,24 +691,27 @@
     </div>
 
     <el-dialog v-model="candidate.visible" title="选择预结算单" width="88%" append-to-body>
-      <el-form
-        :model="candidate.query"
-        inline
-        label-position="right"
-        label-width="88px"
-        class="formal-editor__candidate-search"
-      >
-        <el-form-item label="预结算单号"
+      <el-card>
+        <el-form
+          :model="candidate.query"
+          inline
+          label-position="right"
+          label-width="88px"
+          class="formal-editor__candidate-search"
+        >
+          <el-form-item label="预结算单号"
           ><el-input v-model="candidate.query.preSettlementNo" clearable
-        /></el-form-item>
-        <el-form-item label="合同编号"
+          /></el-form-item>
+          <el-form-item label="合同编号"
           ><el-input v-model="candidate.query.contractNo" clearable
-        /></el-form-item>
-        <el-form-item
+          /></el-form-item>
+          <el-form-item
           ><el-button @click="resetCandidates">重置</el-button
           ><el-button type="primary" @click="loadCandidates">查询</el-button></el-form-item
-        >
-      </el-form>
+          >
+        </el-form>
+      </el-card>
+
       <el-table
         ref="candidateTable"
         v-loading="candidate.loading"