Limit Epic name filter dropdown to a single workspace

Add an option for dashboard filters so that the Epic name dropdown can be restricted to a single specified workspace. The user wants the Epic name filter to:

• Show only epics from one workspace (for example, a specific portfolio workspace) so that epics from other workspaces do not appear as choices.
• Optionally enforce that there is always only one epic selected while limiting the available epics to that workspace.

This would allow more controlled, workspace-specific epic selection in dashboard filters and avoid showing irrelevant epics from other workspaces.