refactor(sample-list): replace layoutManager singleton with lazy accessor

Convert the eager layoutManager singleton to getLayoutManager() (+ __resetLayoutManager
for tests), so its persisted layout preference is read on first access rather than at
module load. Update the model barrels and consumers (LayoutSwitch, SampleListSection,
SampleList) with $derived reads; the LayoutSwitch test resolves via the accessor.

src/widgets/SampleList/ui/LayoutSwitch/LayoutSwitch.svelte

@@ -7,7 +7,7 @@ import { ButtonGroup } from '$shared/ui';
 import { IconButton } from '$shared/ui';
 import GridIcon from '@lucide/svelte/icons/layout-grid';
 import ListIcon from '@lucide/svelte/icons/stretch-horizontal';
-import { layoutManager } from '../../model';
+import { getLayoutManager } from '../../model';
 
 interface Props {
     /**
@@ -18,18 +18,21 @@ interface Props {
 
 const { class: className }: Props = $props();
 
+const layoutManager = getLayoutManager();
+const mode = $derived(layoutManager.mode);
+
 function handleClick() {
     layoutManager.toggleMode();
 }
 </script>
 
 <ButtonGroup class={className}>
-    <IconButton active={layoutManager.mode === 'list'} onclick={handleClick}>
+    <IconButton active={mode === 'list'} onclick={handleClick}>
         {#snippet icon()}
             <ListIcon class="size-4" />
         {/snippet}
     </IconButton>
-    <IconButton active={layoutManager.mode === 'grid'} onclick={handleClick}>
+    <IconButton active={mode === 'grid'} onclick={handleClick}>
         {#snippet icon()}
             <GridIcon class="size-4" />
         {/snippet}