add @ mentions in chat

frontend/src/routes/api/chat/+server.ts

@@ -34,10 +34,11 @@ export async function POST(event: RequestEvent) {
 				? await s.chat.typing(famId, actor, { typing: Boolean(body.typing) })
 				: await s.chat.send(famId, actor, {
 						content: body.content || '',
-						clientId: body.clientId || ''
+						clientId: body.clientId || '',
+						siteUrl: event.url.origin
 					});
 		return json(data);
 	} catch (e) {
 		return json({ error: e instanceof Error ? e.message : 'chat request failed' }, { status: 400 });
 	}
-}
+}