Fix hold countdown not updating for clients connecting mid-hold

HoldRemaining now calls updateHeaderCountdown() directly so the header
hold row refreshes every second from its own message, not only via the
Elapsed → updateStepPlates → updateHeaderCountdown chain.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SH2D4LRuCnhLSzoYerAfJX

web/app.js

@@ -733,6 +733,7 @@ function onSudChanged(msg) {
 		} else if (key === 'HoldRemaining') {
 			sudHoldRemaining = msg.HoldRemaining;
 			updateStatusLine();
+			updateHeaderCountdown();
 		} else if (key === 'Elapsed') {
 			elapsed = msg.Elapsed;
 			updateStepPlates();