Fix Frankfurter integration: require namespace, use v1 API, show converted BTC prices

- Require frankfurter.rates namespace in core.clj so Integrant can find it
- Use /v1/ prefix for self-hosted Frankfurter API
- Display BTC price converted to EUR and DKK instead of raw exchange rates
- Remove unused host port mapping for Frankfurter container

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

docker-compose.yml

@@ -17,5 +17,3 @@ services:
   frankfurter:
     image: lineofflight/frankfurter
     restart: unless-stopped
-    ports:
-      - "8080:8080"