Remove widget system — fully replaced by fragment composition

Delete shared/contracts/widgets.py, shared/services/widget_registry.py,
and shared/services/widgets/ (empty stubs). Remove register_all_widgets()
from factory and widgets Jinja global from jinja_setup. Zero consumers
remain — all cross-app UI composition now uses the fragment API.

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

shared/infrastructure/factory.py

@@ -67,9 +67,6 @@ 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,