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).

static/css/components/header.css

@@ -83,6 +83,12 @@
   white-space: nowrap;
 }
 
+/* Opt-in pages (e.g. Other Models) hide their nav entry until enabled.
+   A class is used instead of [hidden] because .nav-item sets display: flex. */
+.nav-item--hidden {
+  display: none;
+}
+
 .nav-item:hover,
 .nav-item:focus-visible {
   background-color: var(--lora-surface-hover, oklch(95% 0.02 256));