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/lib/stores/theme.ts

@@ -26,16 +26,30 @@ themeMode.subscribe((value) => {
 	}
 });
 
+/**
+ * Tracks system color-scheme preference changes so that the derived store
+ * re-evaluates when the OS theme changes while mode is 'system'.
+ */
+const systemDark = writable(
+	typeof window !== 'undefined'
+		? window.matchMedia('(prefers-color-scheme: dark)').matches
+		: false
+);
+
+if (typeof window !== 'undefined') {
+	const mq = window.matchMedia('(prefers-color-scheme: dark)');
+	mq.addEventListener('change', (e) => {
+		systemDark.set(e.matches);
+	});
+}
+
 /**
  * Resolved theme based on mode and system preference.
  * Returns 'light' or 'dark'.
  */
-export const resolvedTheme = derived(themeMode, ($mode) => {
+export const resolvedTheme = derived([themeMode, systemDark], ([$mode, $dark]) => {
 	if ($mode === 'system') {
-		if (typeof window !== 'undefined') {
-			return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
-		}
-		return 'light';
+		return $dark ? 'dark' : 'light';
 	}
 	return $mode;
 });