Make dashboards follow selected Prometheus datasource

[alzex3]

This PR fixes namespace and cluster dashboard behavior when switching Prometheus datasources.

It standardizes datasource usage so the selected Datasource dropdown drives:

• the namespace variable query (namespace list updates when datasource changes)
• all panel/target Prometheus datasources (metrics switch to the selected datasource)
• the Alert list panel datasource (now follows the selected datasource as well, where applicable)

Also removes legacy import-time datasource placeholders and hardcoded datasource UIDs that made runtime switching inconsistent.

Fixes: #16