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/routes/+page.svelte

@@ -8,21 +8,27 @@
 	<title>Web App Launcher</title>
 </svelte:head>
 
-<main class="flex min-h-screen items-center justify-center bg-background text-foreground">
+<div class="flex min-h-[60vh] items-center justify-center p-6">
 	<div class="text-center">
-		<h1 class="text-4xl font-bold">Web App Launcher</h1>
+		<h1 class="text-4xl font-bold text-foreground">Web App Launcher</h1>
 		{#if data.user}
 			<p class="mt-4 text-muted-foreground">
 				Welcome, {data.user.displayName}. No default board is configured yet.
 			</p>
-			<form method="POST" action="/auth/logout" class="mt-6">
-				<button
-					type="submit"
-					class="rounded-md bg-secondary px-4 py-2 text-sm font-medium text-secondary-foreground hover:bg-secondary/80"
+			<div class="mt-6 flex items-center justify-center gap-3">
+				<a
+					href="/boards"
+					class="rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground hover:bg-primary/90"
 				>
-					Sign Out
-				</button>
-			</form>
+					View Boards
+				</a>
+				<a
+					href="/apps"
+					class="rounded-md border border-border px-4 py-2 text-sm font-medium text-foreground hover:bg-accent"
+				>
+					Browse Apps
+				</a>
+			</div>
 		{/if}
 	</div>
-</main>
+</div>