fix(shifts): expose ExternalUserId in ShiftSignupDto to fix frontend signup state

frontend/src/app/(protected)/shifts/[id]/page.tsx

@@ -23,7 +23,7 @@ export default function ShiftDetailPage({ params }: { params: Promise<{ id: stri
   const capacityPercentage = (shift.signups.length / shift.capacity) * 100;
   const isFull = shift.signups.length >= shift.capacity;
   const isPast = new Date(shift.startTime) < new Date();
-  const isSignedUp = shift.signups.some((s) => s.memberId === session?.user?.id);
+  const isSignedUp = shift.signups.some((s) => s.memberId === session?.user?.id || s.externalUserId === session?.user?.id);
 
   const handleSignUp = async () => {
     await signUpMutation.mutateAsync(shift.id);

frontend/src/hooks/useShifts.ts

@@ -36,6 +36,7 @@ export interface ShiftDetailDto {
 export interface ShiftSignupDto {
   id: string;
   memberId: string;
+  externalUserId?: string;
   signedUpAt: string;
 }