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/c4game/routes.go

@@ -0,0 +1,29 @@
+package c4game
+
+import (
+	"github.com/alexedwards/scs/v2"
+	"github.com/go-chi/chi/v5"
+	"github.com/nats-io/nats.go"
+	"github.com/ryanhamamura/c4/db"
+	"github.com/ryanhamamura/c4/game"
+)
+
+func SetupRoutes(
+	router chi.Router,
+	store *game.GameStore,
+	nc *nats.Conn,
+	sessions *scs.SessionManager,
+	chatPersister *db.ChatPersister,
+) error {
+	router.Get("/game/{game_id}", HandleGamePage(store, sessions, chatPersister))
+	router.Get("/game/{game_id}/events", HandleGameEvents(store, nc, sessions, chatPersister))
+
+	router.Route("/api/game/{game_id}", func(r chi.Router) {
+		r.Post("/drop", HandleDropPiece(store, sessions))
+		r.Post("/chat", HandleSendChat(store, nc, sessions, chatPersister))
+		r.Post("/join", HandleSetNickname(store, sessions))
+		r.Post("/rematch", HandleRematch(store, sessions))
+	})
+
+	return nil
+}