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

backend/internal/service/admin_service_bulk_update_test.go

@@ -105,3 +105,51 @@ func TestAdminService_BulkUpdateAccounts_PartialFailureIDs(t *testing.T) {
 	require.ElementsMatch(t, []int64{2}, result.FailedIDs)
 	require.Len(t, result.Results, 3)
 }
+
+func TestAdminService_BulkUpdateAccounts_NilGroupRepoReturnsError(t *testing.T) {
+	repo := &accountRepoStubForBulkUpdate{}
+	svc := &adminServiceImpl{accountRepo: repo}
+
+	groupIDs := []int64{10}
+	input := &BulkUpdateAccountsInput{
+		AccountIDs: []int64{1},
+		GroupIDs:   &groupIDs,
+	}
+
+	result, err := svc.BulkUpdateAccounts(context.Background(), input)
+	require.Nil(t, result)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "group repository not configured")
+}
+
+// TestAdminService_BulkUpdateAccounts_MixedChannelPreCheckBlocksOnExistingConflict verifies
+// that the global pre-check detects a conflict with existing group members and returns an
+// error before any DB write is performed.
+func TestAdminService_BulkUpdateAccounts_MixedChannelPreCheckBlocksOnExistingConflict(t *testing.T) {
+	repo := &accountRepoStubForBulkUpdate{
+		getByIDsAccounts: []*Account{
+			{ID: 1, Platform: PlatformAntigravity},
+		},
+		// Group 10 already contains an Anthropic account.
+		listByGroupData: map[int64][]Account{
+			10: {{ID: 99, Platform: PlatformAnthropic}},
+		},
+	}
+	svc := &adminServiceImpl{
+		accountRepo: repo,
+		groupRepo:   &groupRepoStubForAdmin{getByID: &Group{ID: 10, Name: "target-group"}},
+	}
+
+	groupIDs := []int64{10}
+	input := &BulkUpdateAccountsInput{
+		AccountIDs: []int64{1},
+		GroupIDs:   &groupIDs,
+	}
+
+	result, err := svc.BulkUpdateAccounts(context.Background(), input)
+	require.Nil(t, result)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "mixed channel")
+	// No BindGroups should have been called since the check runs before any write.
+	require.Empty(t, repo.bindGroupsCalls)
+}