Add widget registry for universal UI decoupling

Introduces a widget system where domains register UI fragments into
named slots (container_nav, container_card, account_page, account_link).
Host apps iterate widgets generically without naming any domain.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infrastructure/factory.py

@@ -65,6 +65,10 @@ def create_base_app(
     """
     if domain_services_fn is not None:
         domain_services_fn()
+
+    from shared.services.widgets import register_all_widgets
+    register_all_widgets()
+
     app = Quart(
         name,
         static_folder=STATIC_DIR,

infrastructure/jinja_setup.py

@@ -101,5 +101,9 @@ def setup_jinja(app: Quart) -> None:
     app.jinja_env.globals["page_cart_url"] = page_cart_url
     app.jinja_env.globals["market_product_url"] = market_product_url
 
+    # widget registry available in all templates
+    from shared.services.widget_registry import widgets as _widget_registry
+    app.jinja_env.globals["widgets"] = _widget_registry
+
     # register jinja filters
     register_filters(app)