refactor(nav): add class prop to BackButton, remove mb-4 from topbar contexts

- BackButton now accepts a `class` prop (default 'mb-4') so callers can
  override spacing; resolves hardcoded margin in flex-row topbar snippets
- documents/[id]/edit and enrich/[id] pass class="" to suppress the margin
- Replace weak className unit test with class-prop behaviour tests
- Add [data-hydrated] comment in E2E spec explaining what emits the attribute

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/edit/+page.svelte

@@ -27,7 +27,7 @@ async function handleDelete() {
 
 <DocumentEditLayout doc={doc} formId="update-form" formAction="?/update" formError={form?.error}>
 	{#snippet topbar()}
-		<BackButton />
+		<BackButton class="" />
 
 		<p class="max-w-sm truncate text-center font-serif text-sm font-medium text-ink">
 			{doc.title || doc.originalFilename}

frontend/src/routes/enrich/[id]/+page.svelte

@@ -15,7 +15,7 @@ const doc = $derived(data.document);
 
 <DocumentEditLayout doc={doc} formId="save-form" formAction="?/save" formError={form?.error}>
 	{#snippet topbar()}
-		<BackButton />
+		<BackButton class="" />
 
 		<p class="max-w-sm truncate text-center font-serif text-sm font-medium text-ink">
 			{doc.title || doc.originalFilename}