Switch to HTMX WebSocket, use LAN IP for btcdata URL

- Replace vanilla JS WebSocket with HTMX ws extension (hx-ext="ws")
- Add HTMX + ws extension CDN scripts to page head
- Zero client-side JavaScript — HTMX handles connection, reconnect, and
  DOM swap of HTML fragments from btcdata
- Hidden div safelists Tailwind classes used in btcdata HTML fragments
- Update BTCDATA_URL to LAN IP (192.168.50.42) for cross-machine access
- Update CLAUDE.md and README.md

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

docker-compose.yml

@@ -8,4 +8,4 @@ services:
     ports:
       - "4041:4040"
     environment:
-      BTCDATA_URL: "http://localhost:4101"
+      BTCDATA_URL: "http://192.168.50.42:4101"