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.spec.ts

@@ -21,9 +21,16 @@ describe('BackButton', () => {
 		backSpy.mockRestore();
 	});
 
-	it('has min-h-[44px] for touch target compliance', async () => {
+	it('applies mb-4 by default', async () => {
 		render(BackButton);
 		const btn = document.querySelector('button');
-		expect(btn?.className).toContain('min-h-[44px]');
+		expect(btn?.className).toContain('mb-4');
+	});
+
+	it('applies custom class prop instead of default', async () => {
+		render(BackButton, { props: { class: 'mr-3 md:hidden' } });
+		const btn = document.querySelector('button');
+		expect(btn?.className).toContain('mr-3');
+		expect(btn?.className).not.toContain('mb-4');
 	});
 });