fix(e2e): fix strict mode violation and conversations sort toggle

- Add exact: true to all language button selectors in lang.spec.ts to
  prevent Playwright from matching "Abmelden" (contains "de") alongside
  the DE language button
- Fix goto in conversations applyFilters to use absolute path
  /conversations?... instead of relative ?... so URL updates correctly
- Set locale: 'de-DE' in playwright.config.ts so Accept-Language header
  is consistent and locale detection defaults to German during tests

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

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

@@ -28,7 +28,7 @@
         if (fromDate) params.set('from', fromDate);
         if (toDate) params.set('to', toDate);
         params.set('dir', sortDir);
-        goto(`?${params.toString()}`, { keepFocus: true });
+        goto(`/conversations?${params.toString()}`, { keepFocus: true });
     }
 
     function toggleSort() {