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/playwright.config.ts

@@ -22,6 +22,7 @@ export default defineConfig({
 
 	use: {
 		baseURL: process.env.E2E_BASE_URL ?? 'http://localhost:3000',
+		locale: 'de-DE', // ensures Accept-Language: de is sent so locale detection defaults to German
 		screenshot: 'on', // always capture screenshots
 		video: 'retain-on-failure',
 		trace: 'retain-on-failure'