fix: folder loading gets error state + gcTime=0 for fresh refetch

Add a proper error branch to the folder tree so a failed sidecar request
shows an error state rather than a perpetual loading spinner. Also sets
gcTime: 0 so the query re-fetches fresh data when the sidebar remounts
instead of holding onto stale cache across navigations.

web/src/lib/components/layout/LeftSidebar.svelte

@@ -83,7 +83,8 @@
 	const foldersQuery = createQuery<PpFolder[]>(() => ({
 		queryKey: ['folders'],
 		queryFn: listFolders,
-		enabled: isAuthenticated()
+		enabled: isAuthenticated(),
+		gcTime: 0
 	}));
 
 	// View counts come from PhotoPrism's `/config` response, which carries a
@@ -736,6 +737,8 @@
 		</div>
 		{#if foldersQuery.isPending}
 			<InlineLoader size="sm" label="Loading folders…" />
+		{:else if foldersQuery.isError}
+			<EmptyState size="compact" tone="destructive" icon={FolderOpen} title="Failed to load folders" description="Try reloading the page." />
 		{:else if !hasSubfolders}
 			<EmptyState size="compact" icon={FolderOpen} title="No subfolders" />
 		{:else if rootExpanded}