Charts page: add an "Unassigned / Not in Dashboard" option to the Dashboards filter

What is the problem or goal you're trying to solve or accomplish?

As workspaces scale, chart hygiene becomes hard to manage. Admins need to find charts that are not tied to any dashboard so they can audit and clean them up, but there is currently no way to isolate unassigned charts. In workspaces with hundreds of charts, identifying which ones are unused and who owns them is heavily manual.

How are you solving it currently?

The Charts page has a "Dashboards" filter, but it only lets you filter by a specific dashboard. there is no "unassigned / none" option. Today the only options are inspecting charts one by one (usually by title), or querying the REST API (GET /api/v1/chart/, which returns each chart's dashboards list) and filtering for charts with an empty dashboards array. The API approach works but is not accessible to non-technical admins.

What is your recommended solution?

Add an "Unassigned / Not in Dashboard" option to the existing Dashboards filter on the Charts page, so admins can instantly isolate charts not tied to any dashboard. Combined with the Owner filter, a user could isolate a specific person's unused charts and share the pre-filtered UI link directly with them, turning the cleanup/audit into a single URL with no external tracking.