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>

Makefile

@@ -0,0 +1,26 @@
+.PHONY: build test vet lint clean all devserver
+
+LDFLAGS := -trimpath -ldflags="-s -w -X main.version=$(shell git describe --tags --always --dirty)"
+
+mcat:
+	go build $(LDFLAGS) -o mcat ./cmd/mcat
+
+build:
+	go build ./...
+
+test:
+	go test ./...
+
+vet:
+	go vet ./...
+
+lint:
+	golangci-lint run ./...
+
+clean:
+	rm -f mcat
+
+all: vet lint test mcat
+
+devserver: mcat
+	./mcat server --config srv/mcat.toml