fix(test): fix login page title test after DGB logo removal

The logo was changed from an SVG to a plain <span>, causing
getByText('Familienarchiv') to match both the logo and the footer.
- Update test to use getByRole('link') for precision
- Remove "De Gruyter" from footer text to align with branding change

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

frontend/src/routes/login/+page.svelte

@@ -47,6 +47,6 @@
 
     <!-- Footer -->
     <div class="py-4 text-center">
-        <p class="text-xs font-sans text-gray-300 uppercase tracking-widest">De Gruyter Familienarchiv</p>
+        <p class="text-xs font-sans text-gray-300 uppercase tracking-widest">Familienarchiv</p>
     </div>
 </div>