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/projects/[id]/+page.svelte

@@ -380,7 +380,7 @@
 						{@const stageInstances = instancesByStage[stage.id] ?? []}
 						<div class="rounded-xl border border-[var(--border-primary)] bg-[var(--surface-card)] shadow-[var(--shadow-sm)] overflow-hidden">
 							<!-- Stage header -->
-							<div class="flex items-center justify-between border-b border-[var(--border-secondary)] px-5 py-4">
+							<div class="flex items-center justify-between flex-wrap gap-2 border-b border-[var(--border-secondary)] px-5 py-4">
 								<div class="flex items-center gap-3 flex-wrap">
 									<h3 class="text-base font-semibold text-[var(--text-primary)]">{stage.name}</h3>
 									<span class="rounded-md bg-[var(--surface-card-hover)] px-2 py-0.5 font-mono text-xs text-[var(--text-tertiary)]">{stage.tag_pattern}</span>