fix: cache auth session on locals to prevent cookies.set after response

The authorization hook already calls locals.auth() which can set cookies.
Layout server loads calling auth() again caused a race where cookies.set()
fired after the response started streaming. Now the hook stashes the session
on locals.session and all layouts reuse it.

src/routes/cospend/+layout.server.ts

@@ -2,6 +2,6 @@ import type { LayoutServerLoad } from "./$types"
 
 export const load : LayoutServerLoad = async ({locals}) => {
 	return {
-		session: await locals.auth()
+		session: locals.session ?? await locals.auth()
 	}
 };