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/+page.svelte

@@ -97,7 +97,7 @@
 			</div>
 		</div>
 		<div class="flex items-center gap-4 rounded-xl border border-[var(--border-primary)] bg-[var(--surface-card)] p-5 shadow-[var(--shadow-sm)]">
-			<div class="flex h-12 w-12 items-center justify-center rounded-xl bg-emerald-50 text-emerald-600">
+			<div class="flex h-12 w-12 items-center justify-center rounded-xl bg-emerald-50 dark:bg-emerald-950/30 text-emerald-600">
 				<IconServer size={24} />
 			</div>
 			<div>
@@ -106,7 +106,7 @@
 			</div>
 		</div>
 		<div class="flex items-center gap-4 rounded-xl border border-[var(--border-primary)] bg-[var(--surface-card)] p-5 shadow-[var(--shadow-sm)]">
-			<div class="flex h-12 w-12 items-center justify-center rounded-xl {totalFailed > 0 ? 'bg-red-50 text-red-600' : 'bg-gray-50 text-gray-400'}">
+			<div class="flex h-12 w-12 items-center justify-center rounded-xl {totalFailed > 0 ? 'bg-red-50 dark:bg-red-950/30 text-red-600' : 'bg-gray-50 dark:bg-gray-800/30 text-gray-400'}">
 				<IconAlert size={24} />
 			</div>
 			<div>