feat(admin): open controller-link settings entries from the settings index

A settings entry with a controller opens that controller's route from
its card (settingsPath); singleton cards still open /settings/<code>.
On a linked controller with no navigation entry of its own the rail
marks Settings current, the breadcrumbs read Settings > entry label
(plus the record title on record and create routes) and the page title
uses the entry label. The settings docs describe the admin behaviour;
the embedded admin shell is rebuilt.

admin/tests/state/useSettings.test.ts

@@ -6,6 +6,8 @@ import {
   settings,
   settingsEntry,
   settingsGroups,
+  settingsLinkFor,
+  settingsPath,
   useSettings,
 } from '../../src/state/useSettings'
 import { settingsFixture } from '../fixtures/typed'
@@ -44,4 +46,22 @@ describe('useSettings (D-21)', () => {
       ['System', ['mail', 'search']],
     ])
   })
+
+  it('links a singleton to its form and a controller link to the controller route', () => {
+    const singleton = settingsFixture.list.data[0]!
+    expect(settingsPath({ ...singleton, code: 'a b' })).toBe('/settings/a%20b')
+    expect(settingsPath({ ...singleton, code: 'locales', model: '', controller: 'acme.demo.locales' })).toBe('/acme/demo/locales')
+    expect(settingsPath({ ...singleton, code: 'broken', model: '', controller: 'acme.demo' })).toBe('/settings')
+    expect(useSettings().settingsPath).toBe(settingsPath)
+  })
+
+  it('finds the link entry that opens a controller, never a singleton', () => {
+    const link = { ...settingsFixture.list.data[0]!, code: 'locales', model: '', controller: 'acme.demo.locales' }
+    setSettings([...settingsFixture.list.data, link])
+    expect(settingsLinkFor('acme.demo.locales')?.code).toBe('locales')
+    expect(settingsLinkFor('acme.demo.widgets')).toBeNull()
+    expect(settingsLinkFor('')).toBeNull()
+    expect(settingsLinkFor(null)).toBeNull()
+    expect(useSettings().settingsLinkFor).toBe(settingsLinkFor)
+  })
 })