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/budget.py

@@ -21,11 +21,12 @@ from app.schemas.budget import (
     BudgetTransactionRead,
 )
 from app.services.budget_expense_control import BudgetExpenseControlModel
+from app.services.budget_pagination import BudgetPaginationMixin
 from app.services.budget_support import BudgetSupportMixin
-from app.services.budget_types import BudgetControlError, SUPPORTED_BUDGET_SUBJECT_CODES
+from app.services.budget_types import BudgetControlError
 
 
-class BudgetService(BudgetSupportMixin):
+class BudgetService(BudgetPaginationMixin, BudgetSupportMixin):
     def __init__(self, db: Session) -> None:
         self.db = db
 
@@ -46,22 +47,13 @@ class BudgetService(BudgetSupportMixin):
         cost_center: str | None = None,
     ) -> list[BudgetAllocationRead]:
         self.ensure_budget_ready()
-        stmt = select(BudgetAllocation).order_by(
-            BudgetAllocation.fiscal_year.desc(),
-            BudgetAllocation.period_key.asc(),
-            BudgetAllocation.department_name.asc(),
-            BudgetAllocation.subject_code.asc(),
-        ).where(BudgetAllocation.subject_code.in_(SUPPORTED_BUDGET_SUBJECT_CODES))
-        if fiscal_year is not None:
-            stmt = stmt.where(BudgetAllocation.fiscal_year == fiscal_year)
-        if period_key:
-            stmt = stmt.where(BudgetAllocation.period_key == period_key)
-        if department_id:
-            stmt = stmt.where(BudgetAllocation.department_id == department_id)
-        if department_name:
-            stmt = stmt.where(BudgetAllocation.department_name == department_name)
-        if cost_center:
-            stmt = stmt.where(BudgetAllocation.cost_center == cost_center)
+        stmt = self.build_allocation_stmt(
+            fiscal_year=fiscal_year,
+            period_key=period_key,
+            department_id=department_id,
+            department_name=department_name,
+            cost_center=cost_center,
+        )
         return [self.serialize_allocation(row) for row in self.db.scalars(stmt).all()]
 
     def get_summary(