fix: merge general improvements from release branch

Backend:
- gateway_handler: pass subject.UserID instead of int64(0) for user-level routing
- setting_handler: add missing BalanceLowNotifyRechargeURL to UpdateSettings response
- openai_gateway_service: use applyAccountStatsCost for account stats pricing integration
- embed_on: add local file override (data/public/) for embedded frontend assets

Frontend:
- useTableSelection: add batchUpdate method for batch operations
- AccountsView: virtual scrolling params, Set-based isSelected, swipe virtualization
- ProxiesView: add batchUpdate to selection and swipe-select
- BulkEditAccountModal: fix submit handler to prevent event object as argument
- SettingsView: move payload construction outside try block
- i18n: add general translation keys (saved, deleted, view, validation, allowUserRefund)
- api/client: reorder error fields for consistency
- stores/payment: clarify pollOrderStatus JSDoc

frontend/src/i18n/locales/en.ts

@@ -247,6 +247,8 @@ export default {
     loading: 'Loading...',
     justNow: 'just now',
     save: 'Save',
+    saved: 'Saved successfully',
+    deleted: 'Deleted successfully',
     cancel: 'Cancel',
     delete: 'Delete',
     edit: 'Edit',
@@ -304,6 +306,7 @@ export default {
     saving: 'Saving...',
     selectedCount: '({count} selected)',
     refresh: 'Refresh',
+    view: 'View',
     settings: 'Settings',
     chooseFile: 'Choose File',
     notAvailable: 'N/A',
@@ -5487,6 +5490,7 @@ export default {
       refundSuccess: 'Refund successful',
       refundInfo: 'Refund Info',
       refundEnabled: 'Refund Enabled',
+      allowUserRefund: 'Allow User Refund',
       alreadyRefunded: 'Already Refunded',
       deductBalance: 'Deduct Balance',
       deductBalanceHint: 'Subtract recharged amount from user balance',
@@ -5556,6 +5560,9 @@ export default {
       tabPlanConfig: 'Plan Configuration',
       tabUserSubs: 'User Subscriptions',
       selectGroup: 'Select a group',
+      groupRequired: 'Please select a subscription group',
+      priceRequired: 'Price must be greater than 0',
+      validityDaysRequired: 'Validity days must be greater than 0',
       groupMissing: 'Missing',
       groupInfo: 'Group Info',
       platform: 'Platform',