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

features/auth/routes.go

@@ -0,0 +1,16 @@
+package auth
+
+import (
+	"github.com/alexedwards/scs/v2"
+	"github.com/go-chi/chi/v5"
+	"github.com/ryanhamamura/c4/db/repository"
+)
+
+func SetupRoutes(router chi.Router, queries *repository.Queries, sessions *scs.SessionManager) error {
+	router.Get("/login", HandleLoginPage())
+	router.Get("/register", HandleRegisterPage())
+	router.Post("/api/auth/login", HandleLogin(queries, sessions))
+	router.Post("/api/auth/register", HandleRegister(queries, sessions))
+
+	return nil
+}