Navigation reflects plugin enable/disable immediately

The nav endpoint now skips disabled plugins (checks the registry), so disabling
a plugin removes it from the menu right away without a restart. Its API routes
remain registered until the next restart (Flask cannot unregister a blueprint
at runtime), but users no longer see links to it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shopdb/core/api/dashboard.py

@@ -120,9 +120,14 @@ def get_navigation():
         {'name': 'Map', 'icon': 'map', 'route': '/map', 'position': 4},
     ])
 
-    # Collect navigation items from all plugins
+    # Collect navigation items from enabled plugins. Disabling persists to the
+    # registry immediately, so a disabled plugin drops out of the menu right
+    # away (its routes stay registered until the next restart - Flask cannot
+    # unregister a blueprint at runtime).
     for name, plugin in pm.get_all_plugins().items():
         try:
+            if not pm.registry.is_enabled(name):
+                continue
             items = plugin.get_navigation_items()
             for item in items:
                 item['plugin'] = name