fix(nav): restore DocumentTopBar back button sizing and add right padding

- BackButton gains showLabel prop: showLabel=false renders icon-only with
  aria-label, no mr-2 on svg (was causing 0px button width in topbar)
- DocumentTopBar: BackButton restored to h-11 w-11 circular touch target
  with showLabel=false matching the original 44×44px <a> it replaced
- Topbar row gets pr-4 (16px right padding per spec); action buttons div
  no longer needs its own pr-3

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

frontend/src/lib/components/BackButton.svelte.spec.ts

@@ -33,4 +33,11 @@ describe('BackButton', () => {
 		expect(btn?.className).toContain('mr-3');
 		expect(btn?.className).not.toContain('mb-4');
 	});
+
+	it('hides label text and sets aria-label when showLabel is false', async () => {
+		render(BackButton, { props: { showLabel: false } });
+		const btn = document.querySelector('button');
+		expect(btn?.textContent?.trim()).toBe('');
+		expect(btn?.getAttribute('aria-label')).toMatch(/zurück/i);
+	});
 });