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

py/routes/other_routes.py

@@ -89,8 +89,8 @@ class OtherRoutes(BaseModelRoutes):
             "standalone_mode": standalone_mode,
         }
         if standalone_mode:
-            # The settings UI cannot edit primary folder_paths, so the empty
-            # state must point at the actual file the user has to edit.
+            # The empty state points at the Model Paths settings section and
+            # shows the settings.json path as a fallback reference.
             context["settings_file"] = getattr(self._settings, "settings_file", "") or ""
         return context