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

router/router.go

@@ -0,0 +1,76 @@
+// Package router wires feature routes and middleware into the central chi mux.
+package router
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"sync"
+
+	"github.com/ryanhamamura/c4/config"
+	"github.com/ryanhamamura/c4/db"
+	"github.com/ryanhamamura/c4/db/repository"
+	"github.com/ryanhamamura/c4/features/auth"
+	"github.com/ryanhamamura/c4/features/c4game"
+	"github.com/ryanhamamura/c4/features/lobby"
+	"github.com/ryanhamamura/c4/features/snakegame"
+	"github.com/ryanhamamura/c4/game"
+	"github.com/ryanhamamura/c4/snake"
+
+	"github.com/alexedwards/scs/v2"
+	"github.com/go-chi/chi/v5"
+	"github.com/nats-io/nats.go"
+	"github.com/starfederation/datastar-go/datastar"
+)
+
+func SetupRoutes(
+	router chi.Router,
+	queries *repository.Queries,
+	sessions *scs.SessionManager,
+	nc *nats.Conn,
+	store *game.GameStore,
+	snakeStore *snake.SnakeStore,
+	chatPersister *db.ChatPersister,
+	assets embed.FS,
+) error {
+	// Static assets
+	subFS, _ := fs.Sub(assets, "assets")
+	router.Handle("/assets/*", http.StripPrefix("/assets/", http.FileServerFS(subFS)))
+
+	// Hot-reload for development
+	if config.Global.Environment == config.Dev {
+		setupReload(router)
+	}
+
+	auth.SetupRoutes(router, queries, sessions)
+	lobby.SetupRoutes(router, queries, sessions, store, snakeStore)
+	c4game.SetupRoutes(router, store, nc, sessions, chatPersister)
+	snakegame.SetupRoutes(router, snakeStore, nc, sessions)
+
+	return nil
+}
+
+func setupReload(router chi.Router) {
+	reloadChan := make(chan struct{}, 1)
+	var hotReloadOnce sync.Once
+
+	router.Get("/reload", func(w http.ResponseWriter, r *http.Request) {
+		sse := datastar.NewSSE(w, r)
+		reload := func() { sse.ExecuteScript("window.location.reload()") } //nolint:errcheck // dev-only
+		hotReloadOnce.Do(reload)
+		select {
+		case <-reloadChan:
+			reload()
+		case <-r.Context().Done():
+		}
+	})
+
+	router.Get("/hotreload", func(w http.ResponseWriter, r *http.Request) {
+		select {
+		case reloadChan <- struct{}{}:
+		default:
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("OK")) //nolint:errcheck // dev-only
+	})
+}