feat: add generic pub/sub helpers and pubsub-crud example

Add typed Publish[T] and Subscribe[T] generic helpers that handle
JSON marshaling, along with vianats.EnsureStream and ReplayHistory
helpers. Refactor nats-chatroom to use the new APIs.

Add pubsub-crud example demonstrating CRUD operations with DaisyUI
toast notifications broadcast to all connected clients via NATS.

pubsub_helpers.go

@@ -0,0 +1,23 @@
+package via
+
+import "encoding/json"
+
+// Publish JSON-marshals msg and publishes to subject.
+func Publish[T any](c *Context, subject string, msg T) error {
+	data, err := json.Marshal(msg)
+	if err != nil {
+		return err
+	}
+	return c.Publish(subject, data)
+}
+
+// Subscribe JSON-unmarshals each message as T and calls handler.
+func Subscribe[T any](c *Context, subject string, handler func(T)) (Subscription, error) {
+	return c.Subscribe(subject, func(data []byte) {
+		var msg T
+		if err := json.Unmarshal(data, &msg); err != nil {
+			return
+		}
+		handler(msg)
+	})
+}