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/skeleton/CardSkeleton.svelte

@@ -0,0 +1,21 @@
+<script lang="ts">
+	interface Props {
+		count?: number;
+	}
+
+	let { count = 1 }: Props = $props();
+
+	const items = $derived(Array.from({ length: count }, (_, i) => i));
+</script>
+
+{#each items as i (i)}
+	<div class="rounded-lg border border-border bg-card p-4">
+		<div class="mb-3 flex items-start justify-between">
+			<div class="skeleton h-10 w-10 rounded-lg"></div>
+			<div class="skeleton h-5 w-14 rounded-full"></div>
+		</div>
+		<div class="skeleton mb-2 h-4 w-3/4 rounded"></div>
+		<div class="skeleton h-3 w-full rounded"></div>
+		<div class="skeleton mt-1 h-3 w-1/2 rounded"></div>
+	</div>
+{/each}