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>

contracts/protocols.py

@@ -78,6 +78,10 @@ class CalendarService(Protocol):
         self, session: AsyncSession, *, user_id: int,
     ) -> list[CalendarEntryDTO]: ...
 
+    async def confirmed_entries_for_posts(
+        self, session: AsyncSession, post_ids: list[int],
+    ) -> dict[int, list[CalendarEntryDTO]]: ...
+
 
 @runtime_checkable
 class MarketService(Protocol):