Implement plugin dynamic settings routes architecture

Plugins can now register their settings pages dynamically via get_settings_routes()
hook, eliminating hardcoded imports that break lean builds.

Backend:
- Add get_settings_routes() hook to BasePlugin (shopdb/plugins/base.py)
- Implement in ComputersPlugin for Operating Systems management
- Add /api/plugins/settings-routes endpoint to expose plugin routes

Frontend:
- ModelCatalogHub fetches plugin routes and dynamically adds them via router.addRoute()
- Tabs render dynamically from pluginTabs ref
- Components lazy-load when clicked

Benefits:
- Lean builds work without manual route exclusions
- Plugins self-contain their UI extensions
- No core code changes needed when adding plugin settings pages
- Follows plugin architecture best practices

Fixes TODO from commit a03fd3de4f

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/router/routes/core.js

@@ -56,13 +56,9 @@ export default [
         name: 'model-catalog-models',
         component: () => import('../../views/settings/ModelsList.vue')
       }
-      // TODO: Operating Systems route should be added by computers plugin, not hardcoded in core
-      // Lean builds without computers plugin fail because this import doesn't exist
-      // {
-      //   path: 'operatingsystems',
-      //   name: 'model-catalog-os',
-      //   component: () => import('../../.plugins-staged/computers/views/OperatingSystemsList.vue')
-      // }
+      // Plugin settings routes (e.g., Operating Systems from computers plugin) are added
+      // dynamically by ModelCatalogHub via router.addRoute() from /api/plugins/settings-routes.
+      // This allows plugins to extend settings hubs without breaking lean builds.
     ]
   },
   {