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

backend/internal/service/admin_service.go

@@ -54,6 +54,7 @@ type AdminService interface {
 	SetAccountError(ctx context.Context, id int64, errorMsg string) error
 	SetAccountSchedulable(ctx context.Context, id int64, schedulable bool) (*Account, error)
 	BulkUpdateAccounts(ctx context.Context, input *BulkUpdateAccountsInput) (*BulkUpdateAccountsResult, error)
+	CheckMixedChannelRisk(ctx context.Context, currentAccountID int64, currentAccountPlatform string, groupIDs []int64) error
 
 	// Proxy management
 	ListProxies(ctx context.Context, page, pageSize int, protocol, status, search string) ([]Proxy, int64, error)
@@ -2114,6 +2115,11 @@ func (s *adminServiceImpl) checkMixedChannelRisk(ctx context.Context, currentAcc
 	return nil
 }
 
+// CheckMixedChannelRisk checks whether target groups contain mixed channels for the current account platform.
+func (s *adminServiceImpl) CheckMixedChannelRisk(ctx context.Context, currentAccountID int64, currentAccountPlatform string, groupIDs []int64) error {
+	return s.checkMixedChannelRisk(ctx, currentAccountID, currentAccountPlatform, groupIDs)
+}
+
 func (s *adminServiceImpl) attachProxyLatency(ctx context.Context, proxies []ProxyWithAccountCount) {
 	if s.proxyLatencyCache == nil || len(proxies) == 0 {
 		return