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>

shopdb/plugins/base.py

@@ -129,6 +129,30 @@ class BasePlugin(ABC):
         """
         return []
 
+    def get_settings_routes(self) -> List[Dict]:
+        """Frontend routes this plugin adds to settings pages.
+
+        Returns a list of route configurations for the plugin's settings pages.
+        Each route is a dict with:
+          hub      - settings hub to attach to ('model-catalog', 'asset-config', etc.)
+          path     - route path relative to hub (e.g. 'operatingsystems')
+          name     - unique route name (e.g. 'model-catalog-os')
+          tab      - tab label shown in hub UI (e.g. 'Operating Systems')
+          component- path to Vue component (e.g. 'plugins/<name>/frontend/views/<Component>.vue')
+
+        Example:
+          return [{
+            'hub': 'model-catalog',
+            'path': 'operatingsystems',
+            'name': 'model-catalog-os',
+            'tab': 'Operating Systems',
+            'component': 'plugins/computers/frontend/views/OperatingSystemsList.vue'
+          }]
+
+        Return [] (default) if the plugin adds no settings pages.
+        """
+        return []
+
     def get_settings_defaults(self) -> List[Dict]:
         """Declare the Setting rows this plugin owns.