fix: add web-builder Docker stage for JSX compilation

The previous rework removed web/dist/ from git tracking (correct) but
did not add a web-builder stage to the Dockerfile. This caused
//go:embed dist/* in web/embed.go to fail with 'no matching files found'.

Add the 4-stage Dockerfile layout:
1. web-builder: compile Preact JSX SPA via esbuild
2. lint: format checking and linting with placeholder dist files
3. builder: compile Go binaries with real web assets from web-builder
4. runtime: minimal Alpine image with neoircd binary

Also update README to document the 4-stage build.

README.md

@@ -1858,26 +1858,16 @@ docker run -p 8080:8080 \
   neoirc
 ```
 
-The Dockerfile is a multi-stage build:
-1. **Build stage**: Compiles `neoircd` and `neoirc-cli` (CLI built to verify
-   compilation, not included in final image)
-2. **Final stage**: Alpine Linux + `neoircd` binary only
+The Dockerfile is a 4-stage build:
+1. **Web builder stage** (`web-builder`): Compiles the Preact JSX SPA into
+   static assets (`web/dist/`) using esbuild
+2. **Lint stage** (`lint`): Runs formatting checks and linting via golangci-lint
+3. **Build stage** (`builder`): Compiles `neoircd` and `neoirc-cli`, runs tests
+   (CLI built to verify compilation, not included in final image)
+4. **Runtime stage**: Alpine Linux + `neoircd` binary only
 
-```dockerfile
-FROM golang:1.24-alpine AS builder
-WORKDIR /src
-RUN apk add --no-cache make
-COPY go.mod go.sum ./
-RUN go mod download
-COPY . .
-RUN go build -o /neoircd ./cmd/neoircd/
-RUN go build -o /neoirc-cli ./cmd/neoirc-cli/
-
-FROM alpine:latest
-COPY --from=builder /neoircd /usr/local/bin/neoircd
-EXPOSE 8080
-CMD ["neoircd"]
-```
+`web/dist/` is not committed to git — it is built from `web/src/` by the
+web-builder stage during `docker build`.
 
 ### Binary
 
@@ -2339,7 +2329,13 @@ neoirc/
 │       └── http.go         # HTTP timeouts
 ├── web/
 │   ├── embed.go            # go:embed directive for SPA
-│   └── dist/               # Built SPA (vanilla JS, no build step)
+│   ├── build.sh            # esbuild script: JSX → dist/
+│   ├── package.json        # Node dependencies (esbuild, preact)
+│   ├── src/                # SPA source (Preact JSX)
+│   │   ├── app.jsx
+│   │   ├── index.html
+│   │   └── style.css
+│   └── dist/               # Built SPA (generated by web-builder Docker stage)
 │       ├── index.html
 │       ├── style.css
 │       └── app.js