feat(frontend): opt-in Other Models toggles, hidden nav and announcement

- The Other nav entry is hidden while the feature is off
  (nav-item--hidden, toggled client-side after enabling) and now uses the
  fa-shapes icon.
- Shared utils/otherModels.js helpers (enable through the settings API,
  open the settings Library section) are reused by the disabled page, the
  announcement banner and the download modal.
- BannerService registers a one-time dismissible "other-models-announcement"
  banner while the feature is off; SettingsManager drops the banner and
  updates the nav when the master switch flips.
- A disabled download routing answer now surfaces a showActionToast with an
  "Enable Other Models" action.
- Settings UI: master toggle + five sub_type checkboxes whose default-root
  selects are disabled when unchecked; i18n keys added to en.json and synced
  (other locales keep TODO placeholders).

templates/components/header.html

@@ -38,9 +38,9 @@
           id="embeddingsNavItem">
           <i class="fas fa-code"></i> <span>{{ t('header.navigation.embeddings') }}</span>
         </a>
-        <a href="/other" class="nav-item{% if current_path.startswith('/other') %} active{% endif %}"
+        <a href="/other" class="nav-item{% if current_path.startswith('/other') %} active{% endif %}{% if not settings.get('enable_other_models') %} nav-item--hidden{% endif %}"
           id="otherNavItem">
-          <i class="fas fa-cubes"></i> <span>{{ t('header.navigation.other') }}</span>
+          <i class="fas fa-shapes"></i> <span>{{ t('header.navigation.other') }}</span>
         </a>
         <a href="/statistics" class="nav-item{% if current_path.startswith('/statistics') %} active{% endif %}"
           id="statisticsNavItem">