fix(frontend): peek and restart take <array-id>[k] and written job IDs

- apiserver peek (parseJobIDParam): <array-id>[k] now resolves to the
  element; it used to drop [k] and look up the array identity itself
  (and panicked in parseArrayName on some inputs). A job number still
  names that job exactly (an element by its own XCE_JOBID); ranges and
  invalid selections are rejected.
- apiserver restart: the decimal-only pre-check (isValidJobId, now
  unused and removed) rejected the written form c-a-ts-seq[k];
  ParseIdentitySelection, already used for the lookup, validates it.
- xcecli jobs restart converts its argument with ToAPIJobID like the
  control commands, so written IDs work and invalid ones fail locally.
- docs: the peek/restart caveat in the Job arrays section.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/frontend/apiserver/controllers/jobs.go

@@ -839,12 +839,6 @@ func (j JobController) RestartJobByID(ctx *gin.Context) {
 	logger.Debug("Receive job restart request:",
 		zap.String("jobId", jobIDStr), zap.String("restarter", restarter))
 
-	if err = isValidJobId(jobIDStr); err != nil {
-		logger.Error(err.Error())
-		ReturnError(ctx, common.INVALID_PARAMS_CODE, common.INVALID_PARAMS_MSG+": "+err.Error(), nil)
-		return
-	}
-
 	// check post body whether correct
 	var req models.JobRestartRequest
 	if err := utils.BindJSONStrict(ctx, &req); err != nil {
@@ -1775,15 +1769,6 @@ func isValidArrayName(arrayName string) error {
 	return nil
 }
 
-func isValidJobId(jobID string) error {
-	matched, _ := regexp.MatchString(`^\d+$`, jobID)
-	matchedIndex, _ := regexp.MatchString(`^\d+\[\d+\]$`, jobID)
-	if !matched && !matchedIndex {
-		return fmt.Errorf("jobid '%s' contains invalid characters. Allowed characters are 0-9 or with index value", jobID)
-	}
-	return nil
-}
-
 func validateAskedHosts(askedHosts string) error {
 	if askedHosts == "" {
 		return nil // Nothing to validate