style(project-apply): 统一弹窗和独立表单底栏按钮风格

- 统一「保存+提交+返回」按钮的视觉层级和排序
- 按次要（默认）、中间（primary plain）、主操作（primary）三档区分按钮样式
- 调整按钮顺序为「辅助→返回/取消→保存草稿→提交」，主操作始终最右
- 修改project-apply、invoice-receipt-form、invoice-application-form三个页面的按钮实现
- 在 MEMORY.md 中新增底栏按钮统一规则说明
- 保留已合规页面按钮风格，避免重复修改

src/views/payment/invoice-receipt-form.vue

@@ -241,14 +241,18 @@
 
       <div class="receipt-form-page__actions">
         <el-button v-if="!readonly" type="primary" plain @click="syncReferenceData">同步</el-button>
-        <el-button v-if="!readonly && canSave" @click="saveDraft">保存</el-button>
+        <el-button @click="goBack">返回</el-button>
+        <el-button
+          v-if="!readonly && canSave"
+          type="primary"
+          plain
+          @click="saveDraft"
+        >保存</el-button>
         <el-button
           v-if="!readonly && canSave && hasPermission('invoice_receipt_submit')"
           type="primary"
           @click="submitForm"
-          >提交</el-button
-        >
-        <el-button @click="goBack">返回</el-button>
+        >提交</el-button>
       </div>
     </el-form>