feat: 完善预算中心图表与确认对话框交互

后端预算服务增加汇总查询和辅助计算，前端预算中心优化趋
势图组件和数据展示，增强确认对话框通用性和样式，完善预
算编辑对话框布局，补充预算端点单元测试。

web/src/assets/styles/views/budget-center-view.css

@@ -546,14 +546,19 @@
 }
 
 .legend-line {
-  width: 18px;
-  height: 0;
-  border-top: 2px dashed #2f7fd7;
+  width: 10px;
+  height: 10px;
+  border-radius: 3px;
+  background: #13a66b;
 }
 
-.legend-line.used {
-  border-top-style: solid;
-  border-top-color: #13a66b;
+.legend-line.occupied {
+  background: #f59e0b;
+}
+
+.legend-line.available {
+  background: #e5edf3;
+  border: 1px solid #cbd5e1;
 }
 
 .budget-chart-panel {
@@ -569,6 +574,41 @@
   padding: 12px 20px 18px;
 }
 
+.budget-alert-empty {
+  min-height: 220px;
+  padding: 28px 24px 30px;
+  display: grid;
+  place-items: center;
+  align-content: center;
+  gap: 10px;
+  text-align: center;
+}
+
+.budget-alert-empty-icon {
+  width: 44px;
+  height: 44px;
+  border-radius: 12px;
+  display: grid;
+  place-items: center;
+  background: #e9f7f1;
+  color: #059669;
+  font-size: 24px;
+}
+
+.budget-alert-empty strong {
+  color: #111827;
+  font-size: 16px;
+  font-weight: 800;
+}
+
+.budget-alert-empty p {
+  max-width: 260px;
+  margin: 0;
+  color: #64748b;
+  font-size: 13px;
+  line-height: 1.6;
+}
+
 .budget-alert-row {
   min-height: 46px;
   display: grid;