UI does not allow selecting namespace for a new cluster and clusters from other namespaces are not displayed

Description

I found two related issues in the Postgres Operator UI.

1. Cannot choose a namespace when creating a cluster

The "Create Cluster" form does not provide any option to select the target Kubernetes namespace. As a result, the generated manifest is created only for the default namespace used by the UI.

2. Clusters created in another namespace are not visible

As a workaround, I generated the manifest in the UI and manually changed the metadata.namespace before applying it.

The PostgreSQL cluster was successfully created and is fully operational in the target namespace. However, it does not appear in the UI's cluster list.

Expected behavior

• The UI should allow selecting the target namespace when creating a new PostgreSQL cluster.
• The cluster list should display clusters from all namespaces that the UI has permission to access, or at least provide a namespace selector/filter.

Actual behavior

• There is no namespace selector during cluster creation.
• Clusters deployed outside the default namespace are not shown in the UI, even though they exist and are managed by the operator.

Start with the Create Cluster form and the cluster-list entry point, tracing how the manifest namespace is chosen and which namespaces are queried for display. Done means users can select a target namespace when creating a cluster and can see accessible clusters from other namespaces, with a namespace filter if needed.