fix(migration): remove unused error fields from PreviewResult

ErrorCount and Errors fields were never populated by PreviewImport,
making the API contract misleading. Remove them from both the Go
struct and the TypeScript interface.

frontend/src/services/migrator/csvMigration.ts

@@ -64,8 +64,6 @@ export interface PreviewTask {
 export interface PreviewResult {
 	tasks: PreviewTask[]
 	total_rows: number
-	error_count: number
-	errors?: string[]
 }
 
 export interface MigrationStatus {

pkg/modules/migration/csv/csv.go

@@ -144,10 +144,8 @@ type PreviewTask struct {
 
 // PreviewResult contains preview data before import
 type PreviewResult struct {
-	Tasks      []PreviewTask `json:"tasks"`
-	TotalRows  int           `json:"total_rows"`
-	ErrorCount int           `json:"error_count"`
-	Errors     []string      `json:"errors,omitempty"`
+	Tasks     []PreviewTask `json:"tasks"`
+	TotalRows int           `json:"total_rows"`
 }
 
 // stripBOM removes the UTF-8 BOM from the beginning of a reader