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/models/job_log.go

@@ -124,7 +124,8 @@ func (r *JobLogRepository) GetAllFiltered(limit, offset int, syncPairID *int64,
 	query := `
 		SELECT
 			j.id, j.sync_pair_id, j.trigger_type, j.status,
-			j.started_at, j.finished_at, j.log_file, j.created_at,
+			j.started_at, j.finished_at, j.log_file,
+			j.error_message, j.error_code, j.created_at,
 			CASE WHEN j.finished_at IS NOT NULL AND j.started_at IS NOT NULL
 				THEN (j.finished_at - j.started_at) ELSE NULL END as duration_seconds,
 			(SELECT COUNT(*) FROM job_logs WHERE job_id = j.id) as log_line_count
@@ -143,10 +144,11 @@ func (r *JobLogRepository) GetAllFiltered(limit, offset int, syncPairID *int64,
 	for rows.Next() {
 		var j JobWithStats
 		var started, finished sql.NullTime
-		var logFile sql.NullString
+		var logFile, errMsg, errCode sql.NullString
 		var durationSeconds sql.NullInt64
 		if err := rows.Scan(&j.ID, &j.SyncPairID, &j.TriggerType, &j.Status,
-			&started, &finished, &logFile, &j.CreatedAt,
+			&started, &finished, &logFile,
+			&errMsg, &errCode, &j.CreatedAt,
 			&durationSeconds, &j.LogLineCount); err != nil {
 			return nil, 0, err
 		}
@@ -159,6 +161,12 @@ func (r *JobLogRepository) GetAllFiltered(limit, offset int, syncPairID *int64,
 		if logFile.Valid {
 			j.LogFile = &logFile.String
 		}
+		if errMsg.Valid {
+			j.ErrorMessage = &errMsg.String
+		}
+		if errCode.Valid {
+			j.ErrorCode = &errCode.String
+		}
 		if durationSeconds.Valid {
 			j.DurationSeconds = &durationSeconds.Int64
 		}