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/layout/MainLayout.svelte

@@ -0,0 +1,67 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import Sidebar from './Sidebar.svelte';
+	import Header from './Header.svelte';
+	import AmbientBackground from '$lib/components/background/AmbientBackground.svelte';
+	import SearchDialog from '$lib/components/search/SearchDialog.svelte';
+	import { ui } from '$lib/stores/ui.svelte.js';
+
+	interface BoardLink {
+		id: string;
+		name: string;
+		icon: string | null;
+	}
+
+	interface UserInfo {
+		displayName: string;
+		email: string;
+		role: string;
+		avatarUrl?: string | null;
+	}
+
+	interface Props {
+		user: UserInfo | null;
+		boards: BoardLink[];
+		children: Snippet;
+	}
+
+	let { user, boards, children }: Props = $props();
+
+	const isAdmin = $derived(user?.role === 'admin');
+</script>
+
+<!-- Ambient Background (fixed, behind everything) -->
+<AmbientBackground />
+
+<div class="relative z-10 flex h-screen overflow-hidden">
+	<!-- Mobile overlay -->
+	{#if ui.isMobile && !ui.sidebarHidden}
+		<button
+			type="button"
+			class="fixed inset-0 z-30 bg-black/50"
+			onclick={() => ui.closeMobileSidebar()}
+			aria-label="Close sidebar"
+		></button>
+	{/if}
+
+	<!-- Sidebar -->
+	{#if !ui.sidebarHidden || !ui.isMobile}
+		<div
+			class="shrink-0 {ui.isMobile ? 'fixed left-0 top-0 z-40 h-full' : 'relative'}"
+		>
+			<Sidebar {boards} {isAdmin} collapsed={ui.isMobile ? false : ui.sidebarCollapsed} />
+		</div>
+	{/if}
+
+	<!-- Main content area -->
+	<div class="flex min-w-0 flex-1 flex-col overflow-hidden">
+		<Header {user} />
+
+		<main class="flex-1 overflow-y-auto">
+			{@render children()}
+		</main>
+	</div>
+</div>
+
+<!-- Search Dialog (modal, z-50) -->
+<SearchDialog />