refactor(client): keep settings constants and schemas off the client contract surfaces

The /client entry of a UI plugin exports no values beyond what cordis
loading needs; the theme constants block returns to type-only re-exports,
the per-namespace schemas move into the shared *-settings modules instead
of widening the host entries, and same-package specs import those internals
directly per the export discipline in packages/client/AGENTS.md.

packages/client/ui-theme/src/theme-settings.ts

@@ -1,5 +1,7 @@
 /** Theme preferences stored in the Host user-settings document. */
 
+import z from 'schemastery'
+
 /** Built-in preferences accepted at the registry and settings boundaries. */
 export const THEME_PREFERENCES = ['light', 'dark', 'system'] as const
 
@@ -21,6 +23,11 @@ export interface ThemeSettings {
   preference: ThemePreference
 }
 
+/** Durable theme schema; also the wire envelope the browser scope validates against. */
+export const ThemeSettingsSchema: z<ThemeSettings> = z.object({
+  [THEME_PREFERENCE_FIELD]: z.union([...THEME_PREFERENCES]).default(DEFAULT_PREFERENCE),
+})
+
 /**
  * Narrow one wire or registry value to a persistable preference.
  * @param value - value crossing the settings or registry boundary.