Migrate to mcdsl: auth, config, csrf, web

- Replace internal/auth with mcdsl/auth
- Replace internal/config with mcdsl/config (embed config.Base)
- Replace internal/webserver/csrf.go with mcdsl/csrf
- Use mcdsl/web for session cookies and template rendering
- Use mcdsl/httpserver for server setup and StatusWriter
- Remove direct mcias client library dependency
- Update .golangci.yaml to v2 format (formatters section)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,18 @@
+FROM golang:1.25-alpine AS builder
+
+WORKDIR /build
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w -X main.version=$(git describe --tags --always --dirty 2>/dev/null || echo unknown)" -o mcat ./cmd/mcat
+
+FROM alpine:3.21
+RUN apk --no-cache add ca-certificates && \
+    adduser -D -h /srv/mcat mcat
+USER mcat
+WORKDIR /srv/mcat
+COPY --from=builder /build/mcat /usr/local/bin/mcat
+VOLUME /srv/mcat
+EXPOSE 8443
+ENTRYPOINT ["mcat"]
+CMD ["server", "--config", "/srv/mcat/mcat.toml"]