fix(e2e): update tests to match current UI and fix panel persistence

Code:
- Persist panelOpen to localStorage so panel stays open after reload
- Auto-open panel to Metadaten when document has no file (no prior state)

Tests:
- Nav active state: check bg-nav-active instead of text-brand-navy
  (nav uses semantic tokens since dark mode refactor)
- Save button: use exact:true to avoid matching "Speichern & abschließen"
  (new button was added alongside the plain "Speichern" button)

Note: annotation tests (documents.spec.ts:324, 356) are pre-existing
flaky failures due to test data contamination, not caused by this PR.

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

frontend/e2e/persons.spec.ts

@@ -212,7 +212,7 @@ test.describe('Conversations', () => {
 	test('nav link is active on the conversations page', async ({ page }) => {
 		await page.goto('/conversations');
 		const navLink = page.getByRole('link', { name: 'Konversationen' });
-		await expect(navLink).toHaveClass(/text-brand-navy/);
+		await expect(navLink).toHaveClass(/bg-nav-active/);
 	});
 
 	test('sort toggle changes the button label', async ({ page }) => {