feat: 社保配置回退SocialYearStandard、月度办理tab切换、员工参保筛选分页

- 后端：dotenv/config 加载 ENCRYPTION_KEY，修复薪资数据解密
- 后端：社保/公积金 calculate API 回退到 SocialYearStandard（通过默认账户）
- 后端：getSocialConfigByMonth/getHousingConfigByMonth 回退到 SocialYearStandard
- 后端：active-declaration 查询条件改为 lte，当月办理的增员也显示在在保列表
- 后端：employee-enrollment API 增加部门/参保状态筛选和分页
- 后端：updateEmployeeSchema 增加 baseSalary/performanceSalary 字段
- 后端：payroll2.routes 批次详情包含 contracts 合同类型
- 前端：月度办理社保/公积金改为 tab 切换（不再同时展开）
- 前端：员工参保列表增加筛选（部门/社保状态/公积金状态）和分页
- 前端：花名册基本信息增加月度工资只读显示（基本+绩效自动计算）
- 前端：薪资批次详情表增加合同类型列

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

backend/package-lock.json

@@ -14,6 +14,7 @@
         "bcryptjs": "^2.4.3",
         "compression": "^1.7.4",
         "cors": "^2.8.5",
+        "dotenv": "^17.4.2",
         "exceljs": "^4.4.0",
         "express": "^4.19.0",
         "express-rate-limit": "^7.4.0",
@@ -1695,6 +1696,18 @@
       "integrity": "sha512-98l0sW87ZT58pU4i61wa2OHwxbiYSbuxsCBozaVnYX2iCnr3bLM3fIes1/ej7h1YdOKuKt/MLs706TVnALA65w==",
       "license": "BSD-2-Clause"
     },
+    "node_modules/dotenv": {
+      "version": "17.4.2",
+      "resolved": "https://registry.npmmirror.com/dotenv/-/dotenv-17.4.2.tgz",
+      "integrity": "sha512-nI4U3TottKAcAD9LLud4Cb7b2QztQMUEfHbvhTH09bqXTxnSie8WnjPALV/WMCrJZ6UV/qHJ6L03OqO3LcdYZw==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://dotenvx.com"
+      }
+    },
     "node_modules/duck": {
       "version": "0.1.12",
       "resolved": "https://registry.npmmirror.com/duck/-/duck-0.1.12.tgz",