Correct what --cron actually suppresses (closes #84)

The Vaultik.UI doc comment claimed the cli layer replaces the writer with
a discarding writer in --cron mode. It does not. UI is built once as
ui.New(os.Stdout) and never reassigned; internal/cli/app.go calls
UI.SetQuiet(true) when --cron or --quiet is set, which drops Begin,
Complete, Info, Notice, Detail, Progress and Banner - but Warningf and
Errorf have no quiet check and are still emitted.

That distinction matters: the end-of-run summary is deliberately routed
through UI.Warningf so cron delivers something, so a reader who believed
the comment would have concluded the opposite of how the code is meant to
work.

The README's --cron description carried the same imprecision ("Silent
unless error") and is corrected alongside it.

Comment and documentation only - the Go diff contains no non-comment
lines, so there is no behavior change.

README.md

@@ -168,7 +168,8 @@ needed.
   (System Settings → Privacy & Security → Full Disk Access) to read
   TCC-protected directories; without it the backup aborts with a permission
   error that explains how to fix it
-* `--cron`: Silent unless error (for crontab)
+* `--cron`: Silent on total success; warnings and errors are still printed
+  (for crontab)
 * `--prune`: After backup, drop older snapshots of each backed-up name and
   remove orphaned blobs from remote storage. By default keeps only the latest
   snapshot per name; use `--keep-newer-than` for a rolling window.