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/snakegame/routes.go

@@ -0,0 +1,22 @@
+package snakegame
+
+import (
+	"github.com/alexedwards/scs/v2"
+	"github.com/go-chi/chi/v5"
+	"github.com/nats-io/nats.go"
+	"github.com/ryanhamamura/c4/snake"
+)
+
+func SetupRoutes(router chi.Router, snakeStore *snake.SnakeStore, nc *nats.Conn, sessions *scs.SessionManager) error {
+	router.Get("/snake/{game_id}", HandleSnakePage(snakeStore, sessions))
+	router.Get("/snake/{game_id}/events", HandleSnakeEvents(snakeStore, nc, sessions))
+
+	router.Route("/api/snake/{game_id}", func(r chi.Router) {
+		r.Post("/dir", HandleSetDirection(snakeStore, sessions))
+		r.Post("/chat", HandleSendChat(snakeStore, nc, sessions))
+		r.Post("/join", HandleSetNickname(snakeStore, sessions))
+		r.Post("/rematch", HandleRematch(snakeStore, sessions))
+	})
+
+	return nil
+}