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/README.md

@@ -12,7 +12,7 @@ Capability interfaces that compiled plugins implement to contribute routes, conf
 
 - Plugin capability interfaces: `pact.HasRoutes`, `pact.HasConfig`, `pact.HasMigrations`, `pact.HasCommands`, `pact.HasModels`, `pact.HasJobs`, `pact.HasSchedule`, `pact.HasLang`, `pact.HasLangOverrides` and `pact.HasMailTemplates`.
 - HTTP contracts: the `pact.Router` group builder (implemented by surf), the `pact.Middleware` type, and named, parameterized (`name:param`) and house-envelope middleware through `pact.HasMiddleware`, `pact.HasMiddlewareFactories` and `pact.HasHouseMiddleware`.
-- Backend registration data: `pact.Permission`, `pact.NavigationItem` and `pact.SettingsItem`, exposed through `pact.HasPermissions`, `pact.HasNavigation` and `pact.HasSettings`.
+- Backend registration data: `pact.Permission`, `pact.NavigationItem` and `pact.SettingsItem`, exposed through `pact.HasPermissions`, `pact.HasNavigation` and `pact.HasSettings`. A settings entry is either a singleton form or a link that opens an admin controller from the Settings page.
 - Admin controller contracts: `pact.AdminController`, `pact.HasAdminControllers`, `pact.AdminAssets` (embedded Winter-shaped admin YAML), `pact.AdminPermissioned` and `pact.AdminRecordSource`.
 - Admin extension contracts, so a plugin extends the compiled admin SPA without a Node build: `pact.AdminClientAssets` (per-controller JS and CSS from the plugin's embedded `assets/` tree, Winter's `addJs`/`addCss`), `pact.HasAdminActions` with `pact.AdminAction`, `pact.AdminActionInput` and `pact.AdminActionResult` (named toolbar and widget actions whose routes, CSRF check, permissions and record scoping the framework owns), `pact.HasAdminBulkActions` with `pact.AdminBulkAction`, `pact.AdminBulkActionInput` and `pact.AdminBulkActionResult` (named actions on the rows selected in a list, which receive records the framework loaded through the list scope, never ids), `pact.HasAdminRecordActions` with `pact.AdminRecordAction`, `pact.AdminRecordActionInput` and `pact.AdminRecordActionResult` (named actions on one record, each with an `Applies` rule for the record's state), and `pact.AdminPartialData` (the curated view model a partial template renders).
 - Optional admin hooks a controller or model can implement: list and form query scoping (`pact.ListExtendQuery`, `pact.FormExtendQuery`), list row states (`pact.ListRowStates` with the fixed `pact.RowState` set `pact.RowStateDeleted`, `pact.RowStateNegative` and `pact.RowStateDisabled`), create, update and delete hooks (`pact.FormBeforeCreate`, `pact.FormAfterUpdate`, `pact.FormBeforeDelete` and their siblings), form-only fields that reach those hooks without being model columns (`pact.FormVirtualFields`), validation rules per operation for admin saves (`pact.FormRules`), relation hooks (`pact.RelationExtendManageQuery`, `pact.RelationExtendOptionsQuery`, `pact.RelationBeforeLink`), relation child hooks around creating, updating and deleting a related record (`pact.RelationBeforeCreate`, `pact.RelationAfterCreate`, `pact.RelationBeforeUpdate`, `pact.RelationAfterUpdate`, `pact.RelationBeforeDelete`, `pact.RelationAfterDelete`), filter scopes (`pact.FilterScope`, `pact.FilterOptions`) and dropdown options (`pact.DropdownOptionsProvider`).
@@ -100,7 +100,7 @@ func (p *Plugin) Schedule() []pact.ScheduledCommand {
 | `pact.HasMailTemplates` | Ships mail templates and layout aliases. |
 | `pact.Permission` | One backend permission entry (code, tab, label, roles). |
 | `pact.NavigationItem` | One backend navigation entry, with an optional side menu. |
-| `pact.SettingsItem` | One settings screen entry. |
+| `pact.SettingsItem` | One settings screen entry: a singleton settings form (`Model`, `Form`, `NewModel`), or a link to an admin controller when `Controller` names its ID, as a WinterCMS `'url' => Backend::url(...)` entry. |
 | `pact.AdminController` | Admin controller identity: ID, model name and YAML config directory. |
 | `pact.AdminAssets` | Embedded tree of the plugin's admin YAML. |
 | `pact.AdminRecordSource` | Supplies a new model record for the generic admin handlers. |

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.