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

sessions/sessions.go

@@ -0,0 +1,31 @@
+// Package sessions configures the SCS session manager backed by SQLite.
+package sessions
+
+import (
+	"database/sql"
+	"log/slog"
+	"net/http"
+	"time"
+
+	"github.com/alexedwards/scs/sqlite3store"
+	"github.com/alexedwards/scs/v2"
+)
+
+// SetupSessionManager creates a configured session manager backed by SQLite.
+// Returns the manager and a cleanup function the caller should defer.
+func SetupSessionManager(db *sql.DB) (*scs.SessionManager, func()) {
+	store := sqlite3store.New(db)
+	cleanup := func() { store.StopCleanup() }
+
+	sessionManager := scs.New()
+	sessionManager.Store = store
+	sessionManager.Lifetime = 30 * 24 * time.Hour
+	sessionManager.Cookie.Path = "/"
+	sessionManager.Cookie.HttpOnly = true
+	sessionManager.Cookie.Secure = false
+	sessionManager.Cookie.SameSite = http.SameSiteLaxMode
+
+	slog.Info("session manager configured")
+
+	return sessionManager, cleanup
+}