feat(quick-261016-0fm): manual theme switch on the rail

- the rail's top button cycles system, light and dark and names the
  current choice; light and dark ignore prefers-color-scheme
- usePreferences applies the cached theme before the first await of boot,
  saves the choice through PUT /account/preferences and reverts with a
  toast when the save fails; me() applies the stored preference

admin/src/app/theme.ts

@@ -1,15 +1,32 @@
-// Colour scheme (D-06, flagged assumption A6): the admin follows the system
-// preference only, with no toggle. The .dark class on the document element
-// swaps the design's dark tokens in main.css; the sidebar stays dark in both
-// modes and gains its #222b40 right border in dark mode.
+// Colour scheme (D-06; assumption A6 superseded). The admin follows the
+// system preference by default; light and dark are manual choices the admin
+// makes on the rail's top button and keeps per account (usePreferences). The
+// .dark class on the document element swaps the design's dark tokens in
+// main.css; the sidebar stays dark in both modes and gains its #222b40 right
+// border in dark mode.
 export const DARK_QUERY = '(prefers-color-scheme: dark)'
 
+/** The admin's theme: follow the system, or always light or dark. */
+export type ThemeChoice = 'system' | 'light' | 'dark'
+
+/** The theme choices in the order the rail button cycles through them. */
+export const THEME_CHOICES: readonly ThemeChoice[] = ['system', 'light', 'dark']
+
+export function isThemeChoice(value: unknown): value is ThemeChoice {
+  return typeof value === 'string' && (THEME_CHOICES as readonly string[]).includes(value)
+}
+
 /**
- * Applies the current preference and follows changes. Returns the
- * unsubscribe function.
+ * Applies a theme choice. light and dark set the class once and subscribe
+ * to nothing; system applies the current preference and follows changes.
+ * Returns the unsubscribe function.
  */
-export function applyColorScheme(win: Window = window): () => void {
+export function applyColorScheme(win: Window = window, choice: ThemeChoice = 'system'): () => void {
   const root = win.document.documentElement
+  if (choice !== 'system') {
+    root.classList.toggle('dark', choice === 'dark')
+    return () => undefined
+  }
   if (typeof win.matchMedia !== 'function') {
     root.classList.remove('dark')
     return () => undefined