fix: decouple lint stage from web-builder in Dockerfile

Remove COPY --from=web-builder from the lint stage so it can run
independently and fail fast. Create placeholder files for the
go:embed directive instead. The build stage still uses the real
SPA assets from web-builder.

Update README to reflect that lint no longer depends on web-builder.

Dockerfile

@@ -15,7 +15,9 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-COPY --from=web-builder /web/dist/ web/dist/
+# Create placeholder files so //go:embed dist/* in web/embed.go resolves
+# without depending on the web-builder stage (lint should fail fast)
+RUN mkdir -p web/dist && touch web/dist/index.html web/dist/style.css web/dist/app.js
 RUN make fmt-check
 RUN make lint