feat(admin): add user balance/concurrency history modal

- Add new API endpoint GET /admin/users/:id/balance-history with pagination and type filter
- Add SumPositiveBalanceByUser for calculating total recharged amount
- Create UserBalanceHistoryModal component with:
  - User info header (email, username, created_at, current balance, notes, total recharged)
  - Type filter dropdown (all/balance/admin_balance/concurrency/admin_concurrency/subscription)
  - Quick deposit/withdraw buttons
  - Paginated history list with icons and colored values
- Add instant tooltip on balance column for better UX
- Add z-index prop to BaseDialog for modal stacking control
- Update i18n translations (zh/en)

frontend/src/components/common/BaseDialog.vue

@@ -4,6 +4,7 @@
       <div
         v-if="show"
         class="modal-overlay"
+        :style="zIndexStyle"
         :aria-labelledby="dialogId"
         role="dialog"
         aria-modal="true"
@@ -60,6 +61,7 @@ interface Props {
   width?: DialogWidth
   closeOnEscape?: boolean
   closeOnClickOutside?: boolean
+  zIndex?: number
 }
 
 interface Emits {
@@ -69,11 +71,17 @@ interface Emits {
 const props = withDefaults(defineProps<Props>(), {
   width: 'normal',
   closeOnEscape: true,
-  closeOnClickOutside: false
+  closeOnClickOutside: false,
+  zIndex: 50
 })
 
 const emit = defineEmits<Emits>()
 
+// Custom z-index style (overrides the default z-50 from CSS)
+const zIndexStyle = computed(() => {
+  return props.zIndex !== 50 ? { zIndex: props.zIndex } : undefined
+})
+
 const widthClasses = computed(() => {
   // Width guidance: narrow=confirm/short prompts, normal=standard forms,
   // wide=multi-section forms or rich content, extra-wide=analytics/tables,