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_handler_test.go

@@ -1470,6 +1470,8 @@ func TestJobHandler_RestartNamedJob(t *testing.T) {
 	}{
 		{"ordinary job by decimal id", strconv.FormatUint(ord, 10), "ordinary"},
 		{"array element by <array-id>[k]", strconv.FormatUint(arr, 10) + "[2]", "elem-2"},
+		{"array element by written <array-id>[k]", backend.FormatJobID(int64(arr)) + "[2]", "elem-2"},
+		{"ordinary job by written id", backend.FormatJobID(int64(ord)), "ordinary"},
 	}
 	for _, c := range cases {
 		before := len(f.submitted)