feat: add declarative Options.Streams for automatic JetStream stream creation

Streams listed in Options.Streams are created by Start() when the
embedded NATS server initializes, replacing manual EnsureStream calls
during setup. Migrates nats-chatroom and pubsub-crud examples.

configuration.go

@@ -53,10 +53,15 @@ type Options struct {
 	// Defaults to "/_datastar.js" if empty.
 	DatastarPath string
 
-	// PubSub enables publish/subscribe messaging. Use vianats.New() for an
-	// embedded NATS backend, or supply any PubSub implementation.
+	// PubSub enables publish/subscribe messaging. When nil, an embedded NATS
+	// server starts automatically in Start(). Supply any PubSub implementation
+	// to replace it.
 	PubSub PubSub
 
+	// Streams declares JetStream streams to create when Start() initializes
+	// the embedded NATS server. Ignored when a custom PubSub is configured.
+	Streams []StreamConfig
+
 	// ContextSuspendAfter is the time a context may be disconnected before
 	// the reaper suspends it (frees page resources but keeps the context
 	// shell for seamless re-init on reconnect). Default: 15m.