docs(selfhost): correct WebSocket guidance for LAN access

The previous note claimed the frontend's auto-derived WebSocket URL
worked on LAN without extra configuration. It does not: Next.js
`rewrites()` only proxy HTTP requests, so the `Upgrade` handshake
required for WebSocket never reaches the Go backend, and real-time
features (chat streaming, live issue updates, notifications) silently
fail when accessing the app via a non-localhost host.

Replace the incorrect sentence with a dedicated subsection that points
users at the reverse-proxy recipe (already documented above, includes
the correct /ws Upgrade headers) and, for setups without a proxy,
documents the build-time NEXT_PUBLIC_WS_URL + selfhost.build.yml
override path.

Refs: GH #1522

SELF_HOSTING_ADVANCED.md

@@ -268,9 +268,25 @@ Then restart the stack:
 docker compose -f docker-compose.selfhost.yml up -d
 ```
 
-The frontend automatically derives the WebSocket URL from the page address, so real-time features (chat streaming, live issue updates, notifications) work over LAN without extra configuration.
+### WebSocket for LAN / Non-localhost Access
 
-> **Note:** If you need to hard-code a different public API / WebSocket endpoint into the web image, use the source-build override: `docker compose -f docker-compose.selfhost.yml -f docker-compose.selfhost.build.yml up -d --build`.
+HTTP requests (issues, comments, uploads) work on LAN out of the box — Next.js rewrites proxy `/api`, `/auth`, and `/uploads` to the backend. **WebSockets do not**: Next.js rewrites only forward HTTP requests, not the `Upgrade` handshake a WebSocket needs. If you open the app on `http://<lan-ip>:3000`, real-time features (chat streaming, live issue updates, notifications) will fail to connect until you do one of the following:
+
+1. **Put a reverse proxy in front of the stack (recommended).** Nginx or Caddy terminates the WebSocket upgrade and forwards it to the backend on port 8080. See the [Reverse Proxy](#reverse-proxy) section above — the Nginx example already includes a `location /ws { ... }` block with the correct `Upgrade` / `Connection` headers. Once a proxy is in place the browser connects directly through it, so no frontend rebuild is needed.
+
+2. **Bake a WebSocket URL into the web image.** If you are not running a reverse proxy, rebuild the web image with `NEXT_PUBLIC_WS_URL` pointing straight at the backend (port 8080 must be reachable from the browser):
+
+   ```bash
+   # In .env
+   NEXT_PUBLIC_WS_URL=ws://<lan-ip>:8080/ws
+
+   # Rebuild the web image so the build-time value is baked in
+   docker compose -f docker-compose.selfhost.yml -f docker-compose.selfhost.build.yml up -d --build
+   ```
+
+   `NEXT_PUBLIC_WS_URL` is a build-time variable (see `Dockerfile.web`), so setting it only in `environment:` on the pre-built image has no effect — you must use the `selfhost.build.yml` override that rebuilds the image.
+
+> **Note:** If you need to hard-code a different public API / WebSocket endpoint into the web image for any other reason, use the same source-build override: `docker compose -f docker-compose.selfhost.yml -f docker-compose.selfhost.build.yml up -d --build`.
 
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