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.

internal/examples/pubsub-crud/main.go

@@ -53,18 +53,14 @@ func main() {
 		DocumentTitle: "Bookmarks",
 		LogLevel:      via.LogLevelInfo,
 		ServerAddress: ":7331",
+		Streams: []via.StreamConfig{{
+			Name:     "BOOKMARKS",
+			Subjects: []string{"bookmarks.>"},
+			MaxMsgs:  1000,
+			MaxAge:   24 * time.Hour,
+		}},
 	})
 
-	err := via.EnsureStream(v, via.StreamConfig{
-		Name:     "BOOKMARKS",
-		Subjects: []string{"bookmarks.>"},
-		MaxMsgs:  1000,
-		MaxAge:   24 * time.Hour,
-	})
-	if err != nil {
-		log.Fatalf("Failed to ensure stream: %v", err)
-	}
-
 	v.AppendToHead(
 		h.Link(h.Rel("stylesheet"), h.Href("https://cdn.jsdelivr.net/npm/daisyui@4/dist/full.min.css")),
 		h.Script(h.Src("https://cdn.tailwindcss.com")),