feat(cabana): read-only Plugins screen with installed plugins

- GET /plugins (backend.view_plugins, superusers always pass) lists the
  plugins from party.Installed, or party.Activated IDs for older builds,
  with the built framework version and whether the registry is configured
- Settings -> System -> Plugins entry with the new SettingsEntry.path,
  added only for principals allowed to open it
- PluginsView at /system/plugins with an Installed tab (reka-ui Tabs);
  settings panel, rail, breadcrumbs and page title know the route
- OpenAPI and SPA types regenerated; contract, token-class and
  conformance inventories list the route; en/pl strings; docs

admin/src/state/useSettings.ts

@@ -1,7 +1,8 @@
 // Settings pages the admin may manage (D-21), server-filtered like the
 // navigation. The rail shows "Ustawienia" only when this list is non-empty.
-// An entry is either a singleton settings form (/settings/<code>) or a link
-// that opens an admin controller (its `controller` is set).
+// An entry is either a singleton settings form (/settings/<code>), a link
+// that opens an admin controller (its `controller` is set) or one of the
+// admin's own screens (its `path` is set, for example /system/plugins).
 import { computed, readonly, ref } from 'vue'
 import { api } from '../api/client'
 import { controllerPath } from '../app/controllerRoutes'
@@ -28,10 +29,14 @@ export function settingsEntry(code: string): SettingsEntry | null {
 }
 
 /**
- * Route a settings entry opens: its controller for a link entry (the index
- * when the ID is malformed), otherwise its singleton form.
+ * Route a settings entry opens: its own SPA path for an admin screen, its
+ * controller for a link entry (the index when the ID is malformed),
+ * otherwise its singleton form.
  */
 export function settingsPath(entry: SettingsEntry): string {
+  if (entry.path) {
+    return entry.path
+  }
   if (entry.controller) {
     return controllerPath(entry.controller) ?? '/settings'
   }
@@ -46,6 +51,11 @@ export function settingsLinkFor(controllerId: string | null): SettingsEntry | nu
   return entries.value.find((entry) => entry.controller !== '' && entry.controller === controllerId) ?? null
 }
 
+/** The entry that opens the admin screen at path (entry.path), if any. */
+export function settingsEntryForPath(path: string): SettingsEntry | null {
+  return entries.value.find((entry) => !!entry.path && entry.path === path) ?? null
+}
+
 /** Entries grouped by category, categories in first-seen order, by order. */
 export const settingsGroups = computed(() => {
   const sorted = entries.value
@@ -97,6 +107,7 @@ export function useSettings() {
     settingsEntry,
     settingsPath,
     settingsLinkFor,
+    settingsEntryForPath,
     settingsSectionOf,
   }
 }