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/__init__.py

@@ -0,0 +1,22 @@
+"""Per-domain widget registration.
+
+Called once at startup after domain services are registered.
+Only registers widgets for domains that are actually available.
+"""
+from __future__ import annotations
+
+
+def register_all_widgets() -> None:
+    from shared.services.registry import services
+
+    if services.has("calendar"):
+        from .calendar_widgets import register_calendar_widgets
+        register_calendar_widgets()
+
+    if services.has("market"):
+        from .market_widgets import register_market_widgets
+        register_market_widgets()
+
+    if services.has("cart"):
+        from .cart_widgets import register_cart_widgets
+        register_cart_widgets()