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/routes/boards/[boardId]/+page.svelte

@@ -0,0 +1,23 @@
+<script lang="ts">
+	import type { PageData } from './$types.js';
+	import Board from '$lib/components/board/Board.svelte';
+	import BoardHeader from '$lib/components/board/BoardHeader.svelte';
+
+	let { data }: { data: PageData } = $props();
+</script>
+
+<svelte:head>
+	<title>{data.board.name}</title>
+</svelte:head>
+
+<div class="mx-auto max-w-7xl px-4 py-6">
+	<BoardHeader
+		name={data.board.name}
+		description={data.board.description}
+		icon={data.board.icon}
+		boardId={data.board.id}
+		canEdit={data.canEdit}
+	/>
+
+	<Board sections={data.board.sections} />
+</div>