fix: address all code review findings

- Extract shared permission logic into boardPermissions.ts utility
- Fix DnD drag revert: add dirty flag to prevent  overwrite
- Wrap OAuth group sync in Prisma transaction (N+1 fix)
- Add empty widgetIds validation in widget reorder API
- Add invalidateAll() after guest toggle PATCH
- Replace console.error with user-visible error banners
- Extract WidgetCreationForm component (DraggableSection was 448 lines)
- Remove unused boardId prop from DraggableSection
- Always include OAuth state parameter + validate in callback
- Clean up copyLink timer on component destroy
- Add type-specific widget config validation in addWidget action

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

@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { t } from 'svelte-i18n';
 	import type { PageData } from './$types.js';
+	import { invalidateAll } from '$app/navigation';
 	import Board from '$lib/components/board/Board.svelte';
 	import BoardHeader from '$lib/components/board/BoardHeader.svelte';
 	import BoardShareDialog from '$lib/components/board/BoardShareDialog.svelte';
@@ -8,16 +9,23 @@
 	let { data }: { data: PageData } = $props();
 
 	let showShareDialog = $state(false);
+	let guestToggleError = $state('');
 
 	async function handleGuestToggle(value: boolean) {
+		guestToggleError = '';
 		try {
-			await fetch(`/api/boards/${data.board.id}`, {
+			const res = await fetch(`/api/boards/${data.board.id}`, {
 				method: 'PATCH',
 				headers: { 'Content-Type': 'application/json' },
 				body: JSON.stringify({ isGuestAccessible: value })
 			});
-		} catch (err) {
-			console.error('Failed to update guest access:', err);
+			if (res.ok) {
+				await invalidateAll();
+			} else {
+				guestToggleError = 'Failed to update guest access';
+			}
+		} catch {
+			guestToggleError = 'Network error updating guest access';
 		}
 	}
 </script>
@@ -37,6 +45,10 @@
 			onShare={() => { showShareDialog = true; }}
 		/>
 
+		{#if guestToggleError}
+			<p class="mb-2 text-sm text-destructive">{guestToggleError}</p>
+		{/if}
+
 		<Board sections={data.board.sections} allApps={data.allApps} />
 	</div>
 </div>