Fix End Day not updating dashboard without refresh

Broadcast is_active in WS timer payload so the frontend can immediately
clear the session when the backend marks it complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/sessions.py

@@ -153,6 +153,7 @@ async def timer_action(
         "session_id": session.id,
         "block_id": event.block_id,
         "current_block_id": session.current_block_id,
+        "is_active": session.is_active,
     }
     await manager.broadcast(session.child_id, ws_payload)