Host activity webhooks: Hosts page gear menu and Activity automations modal

[nulmete]

Related user story

#40493

Task

On the Hosts page, group the existing "Custom host vitals" and "Enroll secrets" header buttons into a gear-icon dropdown with a third option, "Activity automations", which opens a modal to enable/disable a per-fleet webhook and set its destination URL. Disabled on "All fleets" and on Fleet Free (with tooltips).

Condition of satisfaction

Gear menu:

• Global admin on a selected fleet sees all three items; "Activity automations" opens the modal.
• "All fleets" selected → "Activity automations" disabled with tooltip; other items unaffected.
• Fleet Free → "Activity automations" disabled with Premium tooltip; Enroll secrets / Custom host vitals behavior identical to today for every role (admin, maintainer, technician, observer; global and fleet-scoped).
• Users with no visible items see no gear button.
• Keyboard and screen-reader accessible (menu focus management per ActionsDropdown).

Modal:

• Shows stored settings on open (enabled + URL) for the selected fleet, including "No fleet".
• Enabling with invalid/empty URL blocks submit with the same inline error style as the global modal; disabling clears the error.
• Save PATCHes only webhook_settings.host_activities_webhook, shows success toast, closes; server 4xx shows error toast.
• Example payload renders a host activity with details.host_id and no activity_id envelope field.
• GitOps mode: inputs disabled with the standard GitOps messaging.
• yarn test passes with new component tests (render states: free, all-fleets, gitops, error, success).