feat(523): Settings -> Data -> Scheduled backups - the verdict first, then destination, schedule, Test, Back up now and recent runs

BackupScheduleSettings.vue leads with whether the data is backed up
(last good backup, or Overdue), then what is wrong (last failure in
words, an S3 secret this SECRET_KEY cannot open, incomplete config),
then the form: daily toggle and UTC hour, folder or S3 bucket, Test
destination with unsaved values, Back up now (polls while running,
re-armed in finally), and the last runs. The secret is write-only.
formatBytes moves to utils/bytes.ts for both views. Restore from
Backup now also takes the .tar, and says to use the CLI on a 413.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/utils/bytes.ts

@@ -0,0 +1,8 @@
+/** "1.4 MB" — a byte count for people. Binary units, one decimal under 10. */
+export function formatBytes(n: number): string {
+  if (n < 1024) return `${n} B`;
+  const units = ["KB", "MB", "GB", "TB"];
+  let v = n / 1024, i = 0;
+  while (v >= 1024 && i < units.length - 1) { v /= 1024; i++; }
+  return `${v.toFixed(v >= 10 || i === 0 ? 0 : 1)} ${units[i]}`;
+}