fix: address load_factor code review findings

- Fix bulk edit: send 0 instead of null/NaN to clear load_factor
- Fix edit modal: explicit NaN check instead of implicit falsy
- Fix create modal: use ?? instead of || for load_factor
- Add load_factor upper limit validation (max 10000)
- Add //go:build unit tag and self-contained intPtrHelper in test
- Add design intent comments on WaitPlan.MaxConcurrency

frontend/src/components/account/BulkEditAccountModal.vue

@@ -1227,7 +1227,9 @@ const buildUpdatePayload = (): Record<string, unknown> | null => {
   }
 
   if (enableLoadFactor.value) {
-    updates.load_factor = loadFactor.value
+    // 空值/NaN/0 时发送 0（后端约定 <= 0 表示清除）
+    const lf = loadFactor.value
+    updates.load_factor = (lf != null && !Number.isNaN(lf) && lf > 0) ? lf : 0
   }
 
   if (enablePriority.value) {