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>

CLAUDE.md

@@ -36,9 +36,9 @@ btcprice's docker-compose includes btcdata via `include: ../btcdata/docker-compo
 - **Framework:** Kit (Integrant-based)
 - **Server:** Undertow
 - **Routing:** Reitit
-- **Templates:** Hiccup + vanilla JS WebSocket
+- **Templates:** Hiccup + HTMX (WebSocket extension)
 - **Styling:** Tailwind CSS v4
-- **Data backend:** btcdata (separate service at BTCDATA_URL, default http://localhost:4100, WebSocket at ws://localhost:4100)
+- **Data backend:** btcdata (separate service at BTCDATA_URL, default http://192.168.50.42:4100)
 - **No database** — all data comes from btcdata
 
 ## Source Layout
@@ -58,6 +58,6 @@ src/clj/pmagnus/btcprice/
     │   └── formats.clj   # Content negotiation
     └── routes/
         ├── api.clj       # /api routes (JSON)
-        ├── ui.clj        # UI routes (HTML + WebSocket)
+        ├── ui.clj        # UI routes (HTML + HTMX WebSocket)
         └── utils.clj     # Route utilities
 ```