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/e2e/back-button.spec.ts

@@ -1,6 +1,9 @@
 import AxeBuilder from '@axe-core/playwright';
 import { test, expect } from '@playwright/test';
 
+// [data-hydrated] is set by +layout.svelte once SvelteKit's client-side hydration is complete.
+// Waiting on it ensures the component is interactive before we interact with it.
+
 test.describe('BackButton — navigation', () => {
 	test('returns to previous page via history when clicked', async ({ page }) => {
 		// Navigate to persons list, then to a person detail