feat(mvp): phase 5 - board, section & widget system

Add board/section/widget CRUD APIs with permission filtering, board view
page with collapsible sections and app widgets in responsive grid, form-based
board editor, and 9 Svelte components (Board, Section, Widget families).

src/lib/components/section/Section.svelte

@@ -0,0 +1,54 @@
+<script lang="ts">
+	import SectionHeader from './SectionHeader.svelte';
+	import SectionCollapsible from './SectionCollapsible.svelte';
+	import WidgetGrid from '$lib/components/widget/WidgetGrid.svelte';
+
+	interface WidgetData {
+		id: string;
+		type: string;
+		order: number;
+		config: string;
+		appId: string | null;
+		app: {
+			id: string;
+			name: string;
+			url: string;
+			icon: string | null;
+			iconType: string;
+			description: string | null;
+			statuses: Array<{ status: string; responseTime: number | null }>;
+		} | null;
+	}
+
+	interface SectionData {
+		id: string;
+		title: string;
+		icon: string | null;
+		order: number;
+		isExpandedByDefault: boolean;
+		widgets: WidgetData[];
+	}
+
+	interface Props {
+		section: SectionData;
+	}
+
+	let { section }: Props = $props();
+
+	let expanded = $state(section.isExpandedByDefault);
+</script>
+
+<div class="rounded-lg border border-gray-700 bg-gray-800/30">
+	<SectionHeader
+		title={section.title}
+		icon={section.icon}
+		{expanded}
+		onToggle={() => (expanded = !expanded)}
+	/>
+
+	<SectionCollapsible {expanded}>
+		<div class="px-4 pb-4">
+			<WidgetGrid widgets={section.widgets} />
+		</div>
+	</SectionCollapsible>
+</div>