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/stubs.py

@@ -93,6 +93,11 @@ class StubCalendarService:
     ) -> list[CalendarEntryDTO]:
         return []
 
+    async def confirmed_entries_for_posts(
+        self, session: AsyncSession, post_ids: list[int],
+    ) -> dict[int, list[CalendarEntryDTO]]:
+        return {}
+
 
 class StubMarketService:
     async def marketplaces_for_container(