Add serverip= config option to pin the domain to a LAN IP

The "unsupported protocol" failure was never about HTTPS: the Vita
resolved the public domain to the WAN IP, the router (no NAT loopback)
answered with its own admin page, and curl reported the resulting
HTTP/0.9 garbage as CURLE_UNSUPPORTED_PROTOCOL. serverip= pins the
hostname to the LAN IP via CURLOPT_RESOLVE, keeping Host/SNI intact
for the reverse proxy.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -37,6 +37,19 @@ This produces `build/vitaImmich.vpk`.
    apikey=your-immich-api-key
    ```
 
+   If your server sits behind a public domain (reverse proxy) and your
+   router does not support NAT loopback, add the server's LAN IP so the
+   domain is pinned to it while on your home network:
+
+   ```
+   server=https://immich.example.com
+   apikey=your-immich-api-key
+   serverip=192.168.1.100
+   ```
+
+   This keeps the Host header and TLS SNI on the domain (so the reverse
+   proxy still routes correctly) but connects to the LAN IP directly.
+
 4. Relaunch the app.
 
 ## Controls