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/lib/components/board/DraggableBoard.svelte

@@ -53,19 +53,25 @@
 	}: Props = $props();
 
 	let sections = $state<SectionData[]>([...initialSections]);
+	let dirty = $state(false);
+	let errorMessage = $state('');
 
-	// Keep local state in sync when parent data changes
+	// Keep local state in sync when parent data changes (skip during drag)
 	$effect(() => {
-		sections = [...initialSections];
+		if (!dirty) {
+			sections = [...initialSections];
+		}
 	});
 
 	const flipDurationMs = 200;
 
 	function handleConsider(e: CustomEvent<{ items: SectionData[] }>) {
+		dirty = true;
 		sections = e.detail.items;
 	}
 
 	async function handleFinalize(e: CustomEvent<{ items: SectionData[] }>) {
+		dirty = true;
 		sections = e.detail.items;
 		const sectionIds = sections.map((s) => s.id);
 
@@ -76,12 +82,15 @@
 				body: JSON.stringify({ sectionIds })
 			});
 		} catch (err) {
-			console.error('Failed to persist section reorder:', err);
+			errorMessage = err instanceof Error ? err.message : 'Failed to persist section reorder';
+		} finally {
+			dirty = false;
 		}
 	}
 
 	async function handleWidgetsUpdate(sectionId: string, widgets: WidgetData[]) {
 		// Update local state
+		dirty = true;
 		sections = sections.map((s) => (s.id === sectionId ? { ...s, widgets } : s));
 
 		const widgetIds = widgets.map((w) => w.id);
@@ -93,11 +102,17 @@
 				body: JSON.stringify({ widgetIds })
 			});
 		} catch (err) {
-			console.error('Failed to persist widget reorder:', err);
+			errorMessage = err instanceof Error ? err.message : 'Failed to persist widget reorder';
+		} finally {
+			dirty = false;
 		}
 	}
 </script>
 
+{#if errorMessage}
+	<p class="mb-2 text-sm text-destructive">{errorMessage}</p>
+{/if}
+
 {#if sections.length === 0}
 	<div class="rounded-xl border border-border bg-card/50 p-8 text-center">
 		<p class="text-muted-foreground">{$t('board.no_sections')}</p>
@@ -113,7 +128,6 @@
 			<div>
 				<DraggableSection
 					{section}
-					{boardId}
 					{apps}
 					onWidgetsUpdate={handleWidgetsUpdate}
 					{addWidgetSectionId}