feat(settings): editable model library paths for standalone mode

Standalone users previously had to hand-edit settings.json to configure
primary folder_paths. Add a standalone-only Model Paths section to the
settings modal:

- Backend exposes standalone_mode, folder_paths (with template placeholder
  values filtered out) and a data-driven folder_path_schema derived from
  OTHER_MODEL_FOLDER_SUBTYPES via GET /api/lm/settings
- The new section renders multi-path editors per model type from the
  schema, with inline enable_other_models / sub-type controls so other
  model types are configured without leaving the tab
- Persistent restart-required cues after a save: nav dot, inline notice
  and a global banner (unique id per change so dismissals don't mute
  future reminders)
- The missing-model-paths startup banner and the Other Models no-paths
  empty state now deep-link into the new section instead of pointing at
  settings.json

tests/frontend/pages/otherDisabledPage.test.js

@@ -25,6 +25,7 @@ describe('Other Models disabled page', () => {
         document.body.innerHTML = [
             '<button id="enableOtherModelsBtn"></button>',
             '<button id="openOtherModelsSettingsBtn"></button>',
+            '<button id="openModelPathsSettingsBtn"></button>',
             '<button id="openSettingsFolderBtn"></button>',
         ].join('');
 
@@ -65,6 +66,27 @@ describe('Other Models disabled page', () => {
         expect(showModal).toHaveBeenCalledWith('settingsModal');
     });
 
+    it('opens the Model Paths settings from the standalone no-folders state', async () => {
+        const showModal = vi.fn();
+        window.modalManager = { showModal };
+
+        const navItem = document.createElement('button');
+        navItem.className = 'settings-nav-item';
+        navItem.dataset.section = 'modelPaths';
+        const navClick = vi.fn();
+        navItem.addEventListener('click', navClick);
+        document.body.appendChild(navItem);
+
+        document.getElementById('openModelPathsSettingsBtn').dispatchEvent(
+            new MouseEvent('click', { bubbles: true }),
+        );
+
+        expect(showModal).toHaveBeenCalledWith('settingsModal');
+
+        await new Promise((resolve) => setTimeout(resolve, 150));
+        expect(navClick).toHaveBeenCalledTimes(1);
+    });
+
     it('reveals the settings.json location from the standalone no-folders state', async () => {
         global.fetch = vi.fn().mockResolvedValue({
             ok: true,