feat(gui): settings panel with locale and theme preferences

Add the browser Settings surface as slot-composed plugins over new
preference services:

- Rename dsh-client-i18n to dsh-client-locale (locale is the domain
  name); LocaleService adds getLocale()/setLocale(id), immutable
  snapshots, a locale/change event, and dsh.locale persistence.
- ThemeService owns the light/dark/system preference (default system),
  resolves system via prefers-color-scheme, publishes theme/change
  snapshots, persists dsh.theme, and no longer touches the DOM;
  ui-layout's ThemePresenter applies resolved snapshots
  (body[data-ds-dark-theme] + alias tokens) and cleans up on dispose.
- ui-sidebar drops the phase-1 settings dropdown/modal; the foot renders
  the new sidebar.settings slot with the column state.
- New ui-settings shell occupies sidebar.settings: foot trigger row and
  the centered 1080x700 panel (figma 501:29947) with 24% mask, close
  button / mask click / Escape all closing, and a 188px nav projected
  from the settings.section list slot it declares. Nav labels are
  registrant-localized; sections re-register on locale change, so the
  ledger version is the shell's only subscription.
- ui-settings-general registers the General section: Permission and
  Tool Call skeletons, live Language (locale menu) and Appearance
  (Light/Dark/System cubes following the persisted preference); its
  slot store mirrors both service snapshots via apply-side listeners.
- ui-settings-models registers the Models nav entry with an empty
  content column.
- Portaled menus pin z-index above modal overlays (a menu anchored
  inside the settings dialog rendered underneath it and was
  unclickable).
- theme/data/list-pen icons in ui-primitives; settings copy ships as
  zh/en dictionaries; fixture manifests gain the settings rows.

scripts/verify-package-readme-model-experience.ts

@@ -59,7 +59,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/client/ui-trajectory': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/ui-workspace': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/ui-theme': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
-  'packages/client/i18n': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
+  'packages/client/locale': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/web': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/examples/agent-spine-demo': { kind: 'indirect', reason: 'The bundle only mounts model-facing child plugins.' },
   'packages/fs/fs': { kind: 'indirect', reason: 'The service interface delegates model rendering to dsh-tool-fs.' },