feat(mvp): phase 7 - UI polish & ambient backgrounds

Add layout system (sidebar, header, main layout), dark/light/system theme
with HSL customization, 3 ambient backgrounds (mesh gradient, particle field,
aurora), Cmd/Ctrl+K search dialog, page transitions, card hover effects,
status pulse animations, skeleton loaders, and responsive design. Polish
all existing pages with consistent theming.

src/lib/components/app/AppCard.svelte

@@ -43,7 +43,7 @@
 	href={app.url}
 	target="_blank"
 	rel="noopener noreferrer"
-	class="group flex flex-col rounded-lg border border-border bg-card p-4 transition-colors hover:border-primary/50 hover:bg-accent/50"
+	class="card-hover group flex flex-col rounded-xl border border-border bg-card p-4 transition-colors hover:border-primary/50"
 	title={app.description ?? app.name}
 >
 	<div class="mb-3 flex items-start justify-between">
@@ -67,7 +67,7 @@
 		<AppHealthBadge status={currentStatus} />
 	</div>
 
-	<h3 class="truncate text-sm font-semibold text-card-foreground group-hover:text-primary">
+	<h3 class="truncate text-sm font-semibold text-card-foreground transition-colors group-hover:text-primary">
 		{app.name}
 	</h3>