Restore --cron warning visibility; show destination on blob upload

--cron used to behave like full quiet mode for everything that
wasn't an error: warnings were swallowed both in the structured
log channel (LevelError gate) and at the snapshot terminus (the
"Finished (with N warnings)" line went through ui.Complete, which
is silenced under SetQuiet). A backup that, say, hit Full Disk
Access permission errors on a handful of files and skipped them
via --skip-errors would exit 0 and emit nothing — the cron job
would never page anyone.

--cron now obeys "silent only on total success":

  * log.Initialize raises the cron/quiet log level from Error to
    Warn so log.Warn output still reaches stdout (and therefore
    cron's mail).
  * The post-backup terminus message switches to ui.Warning when
    WarningCount > 0. Warning is not silenced by SetQuiet, so cron
    delivers the summary line whenever the count is non-zero. The
    no-warnings path keeps ui.Complete, which IS silenced under
    cron — that's the success path.

Separately, blob upload UI now names the actual destination
instead of the generic "backup destination store" string. The
Begin/Info lines emit the storer's reported Location (s3://bucket,
file:///mnt/usb/backup, rclone://remote/path, etc.), so anyone
watching a backup can see exactly where each blob is landing.

internal/log/log.go

@@ -46,8 +46,12 @@ func Initialize(cfg Config) {
 	var level slog.Level
 
 	if cfg.Cron || cfg.Quiet {
-		// In quiet/cron mode, only show errors
-		level = slog.LevelError
+		// In cron/quiet mode keep warnings and errors visible — the
+		// whole point of --cron is to stay silent only on total
+		// success, so that anything cron emails to root is genuinely
+		// "something went wrong, look at it." A backup with stuck
+		// permission errors or skipped files should NOT be silent.
+		level = slog.LevelWarn
 	} else if cfg.Debug || strings.Contains(os.Getenv("GODEBUG"), "vaultik") {
 		level = slog.LevelDebug
 	} else if cfg.Verbose {