fix: use i18n for mixed-channel warning messages and improve bulk pre-check

- BulkUpdate handler: add structured details to 409 response
- BulkUpdateAccounts: simplify to global pre-check before any DB write;
  remove per-account snapshot tracking which is no longer needed
- MixedChannelError.Error(): restore English message for API compatibility
- BulkEditAccountModal: use t() with details for both pre-check and 409
  fallback paths instead of displaying raw backend strings
- Update test to verify pre-check blocks on existing group conflicts

frontend/src/components/account/BulkEditAccountModal.vue

@@ -1129,7 +1129,11 @@ const preCheckMixedChannelRisk = async (built: Record<string, unknown>): Promise
     if (!result.has_risk) return true
 
     pendingUpdatesForConfirm.value = built
-    mixedChannelWarningMessage.value = result.message || t('admin.accounts.bulkEdit.failed')
+    mixedChannelWarningMessage.value = t('admin.accounts.mixedChannelWarning', {
+      groupName: result.details?.group_name,
+      currentPlatform: result.details?.current_platform,
+      otherPlatform: result.details?.other_platform
+    })
     showMixedChannelWarning.value = true
     return false
   } catch (error: any) {
@@ -1203,7 +1207,11 @@ const submitBulkUpdate = async (baseUpdates: Record<string, unknown>) => {
     // 兜底：多平台混合场景下，预检查跳过，由后端 409 触发确认框
     if (error.status === 409 && error.error === 'mixed_channel_warning') {
       pendingUpdatesForConfirm.value = baseUpdates
-      mixedChannelWarningMessage.value = error.message
+      mixedChannelWarningMessage.value = t('admin.accounts.mixedChannelWarning', {
+        groupName: error.details?.group_name,
+        currentPlatform: error.details?.current_platform,
+        otherPlatform: error.details?.other_platform
+      })
       showMixedChannelWarning.value = true
     } else {
       appStore.showError(error.message || t('admin.accounts.bulkEdit.failed'))