With the Briefwechsel view gone, lib/conversation/ held a single shared
component whose only consumer is lib/document/ThumbnailRow. Move it (and
its spec) into lib/document/, update the import, delete the now-empty
lib/conversation/ folder, and fix the stale frontend/CLAUDE.md lib map.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>