feat: 统一后端分页查询与前端服务层适配

后端新增通用分页模块，为报销单、员工、预算、agent 资产等
端点统一接入分页参数和游标查询，优化 repository 层分页实
现，前端服务层适配分页响应结构，完善预算图表和全局样式，
优化侧边栏和企业选择器组件，引入 Element Plus 插件注册。

server/src/app/services/employee_pagination.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from sqlalchemy.orm import Session
+
+from app.core.logging import get_logger
+from app.schemas.employee import EmployeeRead
+from app.services.employee import EmployeeService
+from app.services.pagination import PageResult
+
+logger = get_logger("app.services.employee")
+
+
+class EmployeePaginationService:
+    def __init__(self, db: Session) -> None:
+        self.service = EmployeeService(db)
+
+    def list_employees_page(
+        self,
+        *,
+        status: str | None = None,
+        keyword: str | None = None,
+        page: int | None,
+        page_size: int | None,
+    ) -> PageResult[EmployeeRead]:
+        self.service.ensure_directory_ready()
+        result = self.service.repository.list_page(
+            status=status,
+            keyword=keyword,
+            page=page,
+            page_size=page_size,
+        )
+        logger.info(
+            "Listed employees page (count=%d, total=%d, page=%d, page_size=%d)",
+            len(result.items),
+            result.total,
+            result.page,
+            result.page_size,
+        )
+        return result.map(self.service._serialize_employee)