refactor: remove SearXNG references and rename binary to kafka

- Rename cmd/searxng-go to cmd/kafka
- Remove all SearXNG references from source comments while keeping
  "SearXNG-compatible API" in user-facing docs
- Update binary paths in README, CLAUDE.md, and Dockerfile
- Update log message to "kafka starting"

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

internal/views/static/css/kafka.css

@@ -1,5 +1,4 @@
 /* kafka — clean, minimal search engine CSS */
-/* Inspired by SearXNG's simple theme class conventions */
 
 :root {
   --color-base: #f5f5f5;