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>

services/widgets/cart_widgets.py

@@ -0,0 +1,15 @@
+"""Cart-domain widgets: orders link on account page."""
+from __future__ import annotations
+
+from shared.contracts.widgets import AccountNavLink
+from shared.services.widget_registry import widgets
+from shared.infrastructure.urls import cart_url
+
+
+def register_cart_widgets() -> None:
+    widgets.add_account_link(AccountNavLink(
+        label="orders",
+        order=100,
+        href_fn=lambda: cart_url("/orders/"),
+        external=True,
+    ))