fix: frontend UX improvements (SSE status, responsive tables, dark mode, login toggle, theme)

- Add SSE connection status banner showing when real-time updates are lost (UX-H8, UX-M1)
- Add password visibility toggle on login page (UX-H10)
- Add dark mode variants to stat card backgrounds (UX-M11)
- Add overflow-x-auto to tables for mobile responsiveness (UX-H9)
- Add flex-wrap to stage header for mobile overflow (UX-H11)
- Fix theme store system preference listener reactivity (UX-M12)
- Parallelize registry health checks (UX-L4)

web/src/routes/+layout.svelte

@@ -43,6 +43,7 @@
 	let healthChecked = $state(false);
 	let healthInterval: ReturnType<typeof setInterval> | null = null;
 	let hintsExpanded = $state(false);
+	let sseConnected = $state(true);
 
 	const dockerConnected = $derived(dockerHealth?.connected ?? false);
 
@@ -90,6 +91,12 @@
 			},
 			onDeployStatus(payload) {
 				instanceStatusStore.notifyDeploy(payload);
+			},
+			onOpen() {
+				sseConnected = true;
+			},
+			onError() {
+				sseConnected = false;
 			}
 		});
 
@@ -278,6 +285,13 @@
 			<span class="text-sm font-bold text-[var(--text-primary)]">{$t('app.name')}</span>
 		</header>
 
+		<!-- SSE connection status banner -->
+		{#if !sseConnected}
+			<div class="bg-amber-500 text-white text-center text-xs py-1 px-4">
+				Real-time connection lost. Reconnecting...
+			</div>
+		{/if}
+
 		<!-- Page content -->
 		<main class="flex-1 overflow-y-auto">
 			<div class="mx-auto max-w-7xl px-4 py-6 sm:px-6 sm:py-8">