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/js/other_disabled.js

@@ -0,0 +1,36 @@
+import { appCore } from './core.js';
+import { showToast } from './utils/uiHelpers.js';
+import { enableOtherModels } from './utils/otherModels.js';
+
+/**
+ * Other Models is an opt-in feature. While it is disabled this page renders an
+ * empty state whose button turns the feature on; the backend then rebuilds the
+ * other-model roots and starts scanning, so a reload lands on the real page.
+ */
+async function handleEnableClick() {
+    const button = document.getElementById('enableOtherModelsBtn');
+    if (!button || button.disabled) return;
+
+    button.disabled = true;
+    try {
+        await enableOtherModels();
+    } catch (error) {
+        button.disabled = false;
+        showToast('other.disabled.enableFailed', { message: error.message }, 'error');
+    }
+}
+
+async function initializeOtherDisabledPage() {
+    // appCore.initialize() wires the shared header (theme, settings modal,
+    // language) so this page is not a dead end.
+    await appCore.initialize();
+
+    const button = document.getElementById('enableOtherModelsBtn');
+    if (button) {
+        button.addEventListener('click', handleEnableClick);
+    }
+}
+
+document.addEventListener('DOMContentLoaded', initializeOtherDisabledPage);
+
+export { handleEnableClick as enableOtherModels, initializeOtherDisabledPage };