Remove the old conversations page that was superseded by briefwechsel/.
No navigation link pointed to /conversations; it was unreachable through
the UI. Deletes 5 files, removes 14 orphaned i18n keys from de/en/es
message bundles, and removes E2E tests that navigated to /conversations.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>