[RFE] Add a new `ui-appco-enabled` setting to control AppCo features in the UI

Is your feature request related to a problem? Please describe.

The UI shows SUSE Application Collection (AppCo) features (for example the app bundle install wizard) in every install, including ones that cannot reach AppCo at all, such as airgap. There is no reliable way for the UI to detect this today, so a new setting is needed to drive it.

The UI work that consumes it is tracked in https://github.com/rancher/dashboard/issues/18364.

Describe the solution you'd like

A new setting added via r/r code, next to the other ui-* settings in https://github.com/rancher/rancher/blob/main/pkg/settings/setting.go:

• Name: ui-appco-enabled
• Default: `` (empty string)

The UI will then show AppCo features as follows:

ui-appco-enabled	UI behaviour
true	always show AppCo
false	never show AppCo
`` (empty, default)	show only if the system-catalog setting is not bundled

With the empty default it works automatically for almost everyone (falling back to the bundled system-catalog state used by airgap installs), while still allowing anyone, airgap or not, to force AppCo on or off.

Describe alternatives you've considered

Detecting an airgap environment automatically from the UI. system-catalog is the best signal we have, but it is an indirect one and it gives no way to opt out in a non-airgap install, so the explicit setting is preferred with that check kept as the fallback.