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/widget/WidgetGrid.svelte

@@ -27,7 +27,7 @@
 </script>
 
 {#if widgets.length === 0}
-	<p class="text-sm text-gray-500">No widgets in this section.</p>
+	<p class="text-sm text-muted-foreground">No widgets in this section.</p>
 {:else}
 	<div class="grid grid-cols-2 gap-3 sm:grid-cols-3 lg:grid-cols-4">
 		{#each widgets as widget (widget.id)}
@@ -35,8 +35,8 @@
 				{#if widget.type === 'app' && widget.app}
 					<AppWidget app={widget.app} />
 				{:else}
-					<div class="flex h-full items-center justify-center rounded-lg border border-gray-700 bg-gray-800/50 p-4">
-						<span class="text-xs text-gray-500">{widget.type} widget</span>
+					<div class="flex h-full items-center justify-center rounded-xl border border-border bg-card p-4">
+						<span class="text-xs text-muted-foreground">{widget.type} widget</span>
 					</div>
 				{/if}
 			</WidgetContainer>