fix: 修复采购规则阈值获取逻辑，支持 amountMax 字段

DEPT_PURCHASE_THRESHOLD 规则配置的是 amountMax=50000，原代码只检查
amountMin 导致规则存在但报错缺失的问题。

src/hooks/usePurchaseRules.ts

@@ -74,13 +74,22 @@ export function usePurchaseRules() {
 
     const getRuleAmount = (code: string, defaultVal?: number) => {
       const rule = rules.value.find(r => r.ruleCode === code);
-      if (!rule || rule.amountMin === undefined || rule.amountMin === null) {
+      if (!rule) {
         if (defaultVal === undefined) {
           throw new Error(`采购规则配置缺失: ${code}，请在系统配置中维护`);
         }
         return defaultVal;
       }
+      if (rule.amountMin !== undefined && rule.amountMin !== null) {
         return Number(rule.amountMin);
+      }
+      if (rule.amountMax !== undefined && rule.amountMax !== null) {
+        return Number(rule.amountMax);
+      }
+      if (defaultVal === undefined) {
+        throw new Error(`采购规则配置缺失: ${code}，请在系统配置中维护`);
+      }
+      return defaultVal;
     };
 
     thresholds.deptPurchase = getRuleAmount('DEPT_PURCHASE_THRESHOLD');