Add job error persistence and friendly error UI

Backend:
- Migration 0003_job_error: adds error_message and error_code columns to jobs table
- models/job.go: add ErrorMessage, ErrorCode fields to Job struct; SetError method; update all SELECT queries
- models/job_log.go: GetAllFiltered also reads error_message and error_code (via Job embed)
- syncengine/engine.go: setJobError() helper; capture errors at Wol timeout (wol_timeout), rsync error (rsync_error), and exit_code failure points
- api/dto.go: add ErrorMessage and ErrorCode to JobResponse
- api/handlers_jobs.go: jobToResp propagates error fields

Frontend:
- api/client.ts: add error_message? and error_code? to Job interface
- lib/status.ts: add ERROR_CODES map with friendly titles/hints; getErrorCodeInfo()
- components/ErrorDetailsModal.tsx: new modal showing error title, hint, full message, job metadata, and stderr log; copy-all and download-log buttons
- pages/JobDetail.tsx: error banner for failed jobs with title/hint; View error button opens ErrorDetailsModal; SSE updates error_message in real-time

internal/api/handlers_jobs.go

@@ -206,11 +206,13 @@ func (h *JobHandler) DownloadLog(w http.ResponseWriter, r *http.Request) {
 
 func jobToResp(j models.Job) JobResponse {
 	resp := JobResponse{
-		ID:          j.ID,
-		SyncPairID:  j.SyncPairID,
-		TriggerType: j.TriggerType,
-		Status:      j.Status,
-		LogFile:     j.LogFile,
+		ID:           j.ID,
+		SyncPairID:   j.SyncPairID,
+		TriggerType:  j.TriggerType,
+		Status:       j.Status,
+		LogFile:      j.LogFile,
+		ErrorMessage: j.ErrorMessage,
+		ErrorCode:    j.ErrorCode,
 	}
 	if j.StartedAt != nil {
 		s := j.StartedAt.Format(time.RFC3339)