fix: blank Heater/Stirrer status on browser disconnect instead of stale text

updateDeviceStatus() gains a tri-state: true/false once the server has
actually reported it, or null for "unknown" - rendered as a blank
'---' badge with both Connect/Disconnect buttons disabled.
setConnected() now resets Heater/Stirrer to null whenever the browser's
own websocket connection drops, so a stale Connected/Disconnected badge
from before the disconnect doesn't keep misleadingly showing as current.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YaPLuRPpyjWcwhMvCvpHCL

web/app.js

@@ -538,16 +538,28 @@ function updateSudActions() {
 // and Connect/Disconnect buttons for one device. Simulated devices are
 // always connected and have nothing to connect/disconnect, so their
 // buttons are hidden entirely rather than just disabled.
+//
+// isConnected is a tri-state: true/false once the server has actually
+// reported it, or null for "unknown" - shown as a blank '---' badge with
+// both buttons disabled, used while the browser itself has no connection
+// to the server at all (see setConnected()) since a stale Connected/
+// Disconnected from before that disconnect would otherwise misleadingly
+// look current.
 function updateDeviceStatus(prefix, isConnected, simulated) {
 	const statusEl = document.getElementById(`${prefix}-status`);
-	statusEl.textContent = isConnected ? 'Connected' : 'Disconnected';
+	if (isConnected === null) {
-	statusEl.className = `panel-status ${isConnected ? 'status-connected' : 'status-disconnected'}`;
+		statusEl.textContent = '---';
+		statusEl.className = 'panel-status status-disconnected';
+	} else {
+		statusEl.textContent = isConnected ? 'Connected' : 'Disconnected';
+		statusEl.className = `panel-status ${isConnected ? 'status-connected' : 'status-disconnected'}`;
+	}
 	const connectBtn = document.getElementById(`btn-${prefix}-connect`);
 	const disconnectBtn = document.getElementById(`btn-${prefix}-disconnect`);
 	connectBtn.classList.toggle('hidden', !!simulated);
 	disconnectBtn.classList.toggle('hidden', !!simulated);
-	connectBtn.disabled = isConnected;
+	connectBtn.disabled = isConnected !== false;
-	disconnectBtn.disabled = !isConnected;
+	disconnectBtn.disabled = isConnected !== true;
 }
 
 // Reflects closedLoop (the Controller panel's Enable toggle) in the
@@ -867,6 +879,20 @@ function setConnected(isConnected) {
 	document.getElementById('lcd-pot-row').style.visibility = isConnected ? 'visible' : 'hidden';
 	document.getElementById('sud-status-line').style.visibility = isConnected ? 'visible' : 'hidden';
 	document.getElementById('env-status-line').style.visibility = isConnected ? 'visible' : 'hidden';
+	if (!isConnected) {
+		// The browser's own link to the server is gone, so whatever
+		// Heater/Stirrer connection state was last reported is now stale -
+		// blank it out rather than leave a misleading Connected/
+		// Disconnected badge showing. Simulated stays unknown too (null,
+		// not false) since updateDeviceStatus() only hides the buttons
+		// once the server says so again.
+		heaterConnected = null;
+		heaterSimulated = null;
+		stirrerConnected = null;
+		stirrerSimulated = null;
+		updateDeviceStatus('heater', heaterConnected, heaterSimulated);
+		updateDeviceStatus('stirrer', stirrerConnected, stirrerSimulated);
+	}
 	updateSudActions();
 }