test(quick-261016-0fm): theme switch, language switcher and footer offset

- theme: light and dark ignore prefers-color-scheme, choices and guard
- i18n: ?locale= only with a preference, availableLocales from meta
- usePreferences: cache validation, cycle order and PUT bodies, revert
  with a danger toast, profile apply, setLocale save, cache and reload
- rail theme button, user menu language group, --shell-offset and the
  form save bar offset; fixture strings and resetState theme reset

admin/tests/app/theme.test.ts

@@ -1,12 +1,12 @@
 import { afterEach, describe, expect, it } from 'vitest'
-import { applyColorScheme, DARK_QUERY } from '../../src/app/theme'
+import { applyColorScheme, DARK_QUERY, isThemeChoice, THEME_CHOICES } from '../../src/app/theme'
 import { fakeMedia } from '../helpers'
 
 afterEach(() => {
   document.documentElement.classList.remove('dark')
 })
 
-describe('applyColorScheme (A6: system preference only)', () => {
+describe('applyColorScheme', () => {
   it('sets .dark from prefers-color-scheme and follows changes until unsubscribed', () => {
     const media = fakeMedia({ [DARK_QUERY]: true })
     const stop = applyColorScheme()
@@ -31,4 +31,41 @@ describe('applyColorScheme (A6: system preference only)', () => {
     expect(document.documentElement.classList.contains('dark')).toBe(false)
     expect(stop()).toBeUndefined()
   })
+
+  it('light never sets .dark and ignores the media query', () => {
+    const media = fakeMedia({ [DARK_QUERY]: true })
+    document.documentElement.classList.add('dark')
+    const stop = applyColorScheme(window, 'light')
+    expect(document.documentElement.classList.contains('dark')).toBe(false)
+    expect(media.listeners(DARK_QUERY)).toBe(0)
+    media.change(DARK_QUERY, true)
+    expect(document.documentElement.classList.contains('dark')).toBe(false)
+    expect(stop()).toBeUndefined()
+  })
+
+  it('dark always sets .dark and ignores the media query', () => {
+    const media = fakeMedia({ [DARK_QUERY]: false })
+    const stop = applyColorScheme(window, 'dark')
+    expect(document.documentElement.classList.contains('dark')).toBe(true)
+    expect(media.listeners(DARK_QUERY)).toBe(0)
+    media.change(DARK_QUERY, false)
+    expect(document.documentElement.classList.contains('dark')).toBe(true)
+    stop()
+  })
+
+  it('dark works without matchMedia', () => {
+    const win = { document } as unknown as Window
+    applyColorScheme(win, 'dark')
+    expect(document.documentElement.classList.contains('dark')).toBe(true)
+  })
+})
+
+describe('theme choices', () => {
+  it('lists system, light and dark in cycle order and guards values', () => {
+    expect(THEME_CHOICES).toEqual(['system', 'light', 'dark'])
+    expect(isThemeChoice('dark')).toBe(true)
+    expect(isThemeChoice('blue')).toBe(false)
+    expect(isThemeChoice(null)).toBe(false)
+    expect(isThemeChoice(1)).toBe(false)
+  })
 })