feat(quick-261022-imo): settings section panel scoped to the current category

- settingsSectionOf returns the current entry's category group (null for one-entry categories)
- SettingsPanel mirrors the plugin side menu, collapse included
- AppShell shows it on settings forms and settings-linked controllers without own navigation
- docs/backend/settings.md describes moving between settings pages

admin/tests/shell/AppShell.test.ts

@@ -4,12 +4,13 @@ import { afterEach, beforeEach, describe, expect, it } from 'vitest'
 import { enableAutoUnmount, flushPromises, mount } from '@vue/test-utils'
 import AppShell from '../../src/components/shell/AppShell.vue'
 import { setNavigation } from '../../src/state/useNavigation'
+import { setSettings } from '../../src/state/useSettings'
 import { NARROW_QUERY, setCollapsed } from '../../src/state/useSidebar'
 import { showToast } from '../../src/state/useToasts'
 import { navigationFixture } from '../fixtures/typed'
 import { setBundle } from '../../src/app/i18n'
 import { fakeMedia, resetState, routerAt, signIn } from '../helpers'
-import { langFixture } from '../fixtures/typed'
+import { langFixture, settingsFixture } from '../fixtures/typed'
 
 async function mountShell(path: string) {
   await signIn()
@@ -134,3 +135,20 @@ describe('View site link (Phase 16.2 quick follow-up)', () => {
     expect((wrapper.element as HTMLElement).style.getPropertyValue('--shell-offset')).toBe('var(--spacing-rail)')
   })
 })
+
+describe('settings section panel', () => {
+  it('lists the current category on a settings form, current entry highlighted', async () => {
+    setSettings(settingsFixture.list.data)
+    const wrapper = await mountShell('/settings/mail')
+    const panel = wrapper.find('aside[data-settings-panel]')
+    expect(panel.exists()).toBe(true)
+    expect(panel.find('span').text()).toBe('System')
+    const links = panel.findAll('[data-settings-panel-item]')
+    expect(links.map((link) => [link.text(), link.attributes('href')])).toEqual([
+      ['Mail', '/settings/mail'],
+      ['Search', '/settings/search'],
+    ])
+    expect(links[0]!.attributes('aria-current')).toBe('page')
+    expect(links[1]!.attributes('aria-current')).toBeUndefined()
+  })
+})