feat(dashboards): expose keyboard mapping popover

Is your feature request related to a problem?

Dashboard keyboard shortcuts are easy to forget or discover. Users who rely on the keyboard have no in-context reference for what is available on the dashboards surface.

Describe the solution you'd like

Expose a keyboard mapping popover (or equivalent affordance—e.g. ? shortcut, footer link, or command palette entry) from the dashboards UI so users can see shortcuts without leaving the page.

Describe alternatives you've considered

• Relying on global app help only (less discoverable on dashboards).
• Static docs link only (context switch).

Additional context

Scoped to dashboards; implementation can align with existing keyboard-help patterns elsewhere in the app if any.