feat(cabana): settings entries that link to an admin controller

pact.SettingsItem gains an additive Controller field, the equivalent of a
WinterCMS registerSettings 'url' => Backend::url(...) entry. A link entry
declares no Model, Form or NewModel and needs no AdminFS; start-up fails
when it combines Controller with a singleton form or a model, or names an
unregistered controller. Settings codes stay one namespace.

GET /settings lists a link entry with its controller only when the
principal passes the item's permissions and may open the controller.
Registry.Setting never returns a link entry, so the singleton settings
endpoints answer 404 for its code. SettingsEntry carries controller,
empty for singletons; the OpenAPI document, generated SPA types and
settings fixture follow, and the pact and cabana READMEs and the
settings docs describe the link.

modules/pact/capabilities.go

@@ -398,7 +398,9 @@ type HasNavigation interface {
 	Navigation() []NavigationItem
 }
 
-// SettingsItem is one registerSettings() entry.
+// SettingsItem is one registerSettings() entry. An item is either a singleton
+// settings form (Model, Form and NewModel) or a link to an admin controller
+// (Controller), never both.
 type SettingsItem struct {
 	Code        string
 	Label       string
@@ -409,8 +411,13 @@ type SettingsItem struct {
 	Order       int
 	Keywords    []string
 	Permissions []string
-	Form        string     `json:"-"`
-	NewModel    func() any `json:"-"`
+	// Controller, when set, makes the entry a link to that admin controller
+	// ID (vendor.plugin.controller): the equivalent of a WinterCMS
+	// registerSettings 'url' => Backend::url(...) entry. A link entry
+	// declares no Model, Form or NewModel and is not a singleton.
+	Controller string
+	Form       string     `json:"-"`
+	NewModel   func() any `json:"-"`
 }
 
 // HasSettings is implemented by plugins that declare settings screens.