feat: add mixed-channel precheck API for account-group binding

Add a dedicated CheckMixedChannel endpoint that allows the frontend
to pre-validate mixed channel risk before submitting create/update
requests. This improves UX by showing warnings earlier in the flow
instead of only after form submission.

Backend changes:
- Add CheckMixedChannelRequest struct and CheckMixedChannel handler
- Register POST /check-mixed-channel route
- Expose CheckMixedChannelRisk as public method on AdminService
- Simplify Create/Update 409 responses (remove details/require_confirmation)
- Add comprehensive handler tests and stub methods

Frontend changes:
- Add checkMixedChannelRisk API function and TypeScript types
- Refactor CreateAccountModal to precheck before step transition and submission
- Refactor EditAccountModal to precheck before update submission
- Replace pendingPayload pattern with action-based dialog flow

frontend/src/api/admin/accounts.ts

@@ -15,7 +15,9 @@ import type {
   AccountUsageStatsResponse,
   TempUnschedulableStatus,
   AdminDataPayload,
-  AdminDataImportResult
+  AdminDataImportResult,
+  CheckMixedChannelRequest,
+  CheckMixedChannelResponse
 } from '@/types'
 
 /**
@@ -133,6 +135,16 @@ export async function update(id: number, updates: UpdateAccountRequest): Promise
   return data
 }
 
+/**
+ * Check mixed-channel risk for account-group binding.
+ */
+export async function checkMixedChannelRisk(
+  payload: CheckMixedChannelRequest
+): Promise<CheckMixedChannelResponse> {
+  const { data } = await apiClient.post<CheckMixedChannelResponse>('/admin/accounts/check-mixed-channel', payload)
+  return data
+}
+
 /**
  * Delete account
  * @param id - Account ID
@@ -535,6 +547,7 @@ export const accountsAPI = {
   getById,
   create,
   update,
+  checkMixedChannelRisk,
   delete: deleteAccount,
   toggleStatus,
   testAccount,