feat: the admin Overview shows who is listening, downloads, the library and its disk, and the server's version, uptime and recent errors; a download waiting on you, a client not answering or a full disk count in its verdict (M504 #5627 #5628 #5629 #5630)

The version moves to internal/buildinfo, so /rest/ping reports the build
instead of "dev".

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

internal/logging/logging.go

@@ -23,7 +23,8 @@ func New(w io.Writer, level, format string) (*slog.Logger, error) {
 	default:
 		return nil, fmt.Errorf("unknown log format %q (want text|json)", format)
 	}
-	return slog.New(h), nil
+	// Errors are also kept for the admin Overview (M504).
+	return slog.New(newRingHandler(h, Errors)), nil
 }
 
 func parseLevel(s string) (slog.Level, error) {