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/lib/components/BackButton.svelte

@@ -1,11 +1,12 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
+let { class: cls = 'mb-4' }: { class?: string } = $props();
 </script>
 
 <button
 	type="button"
 	onclick={() => history.back()}
-	class="group mb-4 inline-flex min-h-[44px] items-center text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors outline-none hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring"
+	class="group {cls} inline-flex min-h-[44px] items-center text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors outline-none hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring"
 >
 	<svg
 		class="mr-2 h-4 w-4 transform transition-transform group-hover:-translate-x-1"