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/market_widgets.py

@@ -0,0 +1,24 @@
+"""Market-domain widgets: marketplace links on container pages."""
+from __future__ import annotations
+
+from shared.contracts.widgets import NavWidget
+from shared.services.widget_registry import widgets
+from shared.services.registry import services
+
+
+async def _nav_markets_context(
+    session, *, container_type, container_id, post_slug, **kw,
+):
+    markets = await services.market.marketplaces_for_container(
+        session, container_type, container_id,
+    )
+    return {"markets": markets, "post_slug": post_slug}
+
+
+def register_market_widgets() -> None:
+    widgets.add_container_nav(NavWidget(
+        domain="market",
+        order=30,
+        context_fn=_nav_markets_context,
+        template="_widgets/container_nav/market_links.html",
+    ))