refactor: replace via framework with chi + templ + datastar

Migrate from the via meta-framework to direct dependencies:
- chi for routing, templ for HTML templates, datastar for SSE/reactivity
- Feature-sliced architecture (features/{auth,lobby,c4game,snakegame}/)
- Shared layouts and components (features/common/)
- Handler factory pattern (HandleX(deps) http.HandlerFunc)
- Embedded NATS server (nats/), SCS sessions (sessions/), chi router wiring (router/)
- Move ChatMessage domain type from ui package to game package
- Remove old ui/ package (gomponents-based via/h views)
- Remove via dependency from go.mod entirely

nats/nats.go

@@ -0,0 +1,69 @@
+// Package nats sets up an embedded NATS server for real-time pub/sub
+// messaging between game clients.
+package nats
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"net"
+	"os"
+	"strconv"
+
+	"github.com/delaneyj/toolbelt"
+	"github.com/delaneyj/toolbelt/embeddednats"
+	natsserver "github.com/nats-io/nats-server/v2/server"
+	"github.com/nats-io/nats.go"
+)
+
+func SetupNATS(ctx context.Context) (*nats.Conn, func(), error) {
+	natsPort, err := getFreeNatsPort()
+	if err != nil {
+		return nil, nil, fmt.Errorf("obtaining NATS port: %w", err)
+	}
+
+	ns, err := embeddednats.New(ctx, embeddednats.WithNATSServerOptions(&natsserver.Options{
+		NoSigs: true,
+		Port:   natsPort,
+	}))
+	if err != nil {
+		return nil, nil, fmt.Errorf("creating embedded nats server: %w", err)
+	}
+
+	ns.WaitForServer()
+	slog.Info("NATS started", "port", natsPort)
+
+	nc, err := ns.Client()
+	if err != nil {
+		return nil, nil, fmt.Errorf("creating nats client: %w", err)
+	}
+
+	cleanup := func() {
+		nc.Close()
+		ns.Close()
+	}
+
+	return nc, cleanup, nil
+}
+
+func getFreeNatsPort() (int, error) {
+	if p, ok := os.LookupEnv("NATS_PORT"); ok {
+		natsPort, err := strconv.Atoi(p)
+		if err != nil {
+			return 0, fmt.Errorf("parsing NATS_PORT: %w", err)
+		}
+		if isPortFree(natsPort) {
+			return natsPort, nil
+		}
+	}
+	return toolbelt.FreePort()
+}
+
+func isPortFree(port int) bool {
+	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
+	if err != nil {
+		return false
+	}
+	ln.Close() //nolint:errcheck // checking port availability
+	return true
+}