fix: bulk edit mixed channel warning not showing confirmation dialog

The response interceptor in client.ts transforms errors into plain
objects {status, code, message}, but catch blocks were checking
error.response?.status (AxiosError format) which never matched.

- Add error field passthrough in client.ts interceptor
- Refactor BulkEditAccountModal to use pre-check API (checkMixedChannelRisk)
  before submit, matching the single edit flow
- Fix EditAccountModal catch blocks to use interceptor error format
- Add bulk-update mixed channel unit tests

backend/internal/handler/admin/account_handler_mixed_channel_test.go

@@ -19,6 +19,7 @@ func setupAccountMixedChannelRouter(adminSvc *stubAdminService) *gin.Engine {
 	router.POST("/api/v1/admin/accounts/check-mixed-channel", accountHandler.CheckMixedChannel)
 	router.POST("/api/v1/admin/accounts", accountHandler.Create)
 	router.PUT("/api/v1/admin/accounts/:id", accountHandler.Update)
+	router.POST("/api/v1/admin/accounts/bulk-update", accountHandler.BulkUpdate)
 	return router
 }
 
@@ -145,3 +146,53 @@ func TestAccountHandlerUpdateMixedChannelConflictSimplifiedResponse(t *testing.T
 	require.False(t, hasDetails)
 	require.False(t, hasRequireConfirmation)
 }
+
+func TestAccountHandlerBulkUpdateMixedChannelConflict(t *testing.T) {
+	adminSvc := newStubAdminService()
+	adminSvc.bulkUpdateAccountErr = &service.MixedChannelError{
+		GroupID:         27,
+		GroupName:       "claude-max",
+		CurrentPlatform: "Antigravity",
+		OtherPlatform:   "Anthropic",
+	}
+	router := setupAccountMixedChannelRouter(adminSvc)
+
+	body, _ := json.Marshal(map[string]any{
+		"account_ids": []int64{1, 2, 3},
+		"group_ids":   []int64{27},
+	})
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/admin/accounts/bulk-update", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	router.ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusConflict, rec.Code)
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
+	require.Equal(t, "mixed_channel_warning", resp["error"])
+	require.Contains(t, resp["message"], "claude-max")
+}
+
+func TestAccountHandlerBulkUpdateMixedChannelConfirmSkips(t *testing.T) {
+	adminSvc := newStubAdminService()
+	router := setupAccountMixedChannelRouter(adminSvc)
+
+	body, _ := json.Marshal(map[string]any{
+		"account_ids":                []int64{1, 2},
+		"group_ids":                  []int64{27},
+		"confirm_mixed_channel_risk": true,
+	})
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/admin/accounts/bulk-update", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	router.ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
+	require.Equal(t, float64(0), resp["code"])
+	data, ok := resp["data"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, float64(2), data["success"])
+	require.Equal(t, float64(0), data["failed"])
+}

backend/internal/handler/admin/admin_service_stub_test.go

@@ -22,9 +22,10 @@ type stubAdminService struct {
 	updatedProxyIDs  []int64
 	updatedProxies   []*service.UpdateProxyInput
 	testedProxyIDs   []int64
-	createAccountErr error
-	updateAccountErr error
-	checkMixedErr    error
+	createAccountErr     error
+	updateAccountErr     error
+	bulkUpdateAccountErr error
+	checkMixedErr        error
 	lastMixedCheck   struct {
 		accountID int64
 		platform  string
@@ -235,7 +236,10 @@ func (s *stubAdminService) SetAccountSchedulable(ctx context.Context, id int64,
 }
 
 func (s *stubAdminService) BulkUpdateAccounts(ctx context.Context, input *service.BulkUpdateAccountsInput) (*service.BulkUpdateAccountsResult, error) {
-	return &service.BulkUpdateAccountsResult{Success: 1, Failed: 0, SuccessIDs: []int64{1}}, nil
+	if s.bulkUpdateAccountErr != nil {
+		return nil, s.bulkUpdateAccountErr
+	}
+	return &service.BulkUpdateAccountsResult{Success: len(input.AccountIDs), Failed: 0, SuccessIDs: input.AccountIDs}, nil
 }
 
 func (s *stubAdminService) CheckMixedChannelRisk(ctx context.Context, currentAccountID int64, currentAccountPlatform string, groupIDs []int64) error {