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/board/Board.svelte

@@ -34,8 +34,8 @@
 
 <div class="space-y-6">
 	{#if sections.length === 0}
-		<div class="rounded-lg border border-gray-700 bg-gray-800/50 p-12 text-center">
-			<p class="text-gray-400">This board has no sections yet.</p>
+		<div class="rounded-xl border border-border bg-card/50 p-12 text-center">
+			<p class="text-muted-foreground">This board has no sections yet.</p>
 		</div>
 	{:else}
 		{#each sections as section (section.id)}