fix(trips): deleting a guest keeps the costs adding up instead of dropping their payments

A guest's payer rows and recorded payments hung on the users row by ON DELETE
CASCADE, so deleting the guest wiped money that had really moved: expenses
they paid were left without a payer, settled payments reopened, and a custom
or receipt split lost the guest's share so the balances no longer summed to
zero.

Deleting a guest who paid for an expense or is part of a recorded payment is
now refused with a 409 (REST and the delete_trip_guest MCP tool, same text),
and the share dialog says to change the payer or delete the payment first.
Otherwise the guest's share is handed on: an equal split re-splits as before,
a custom amount goes to the others by largest remainder in the currency's
smallest step (members without an amount get one, so the expense stays a
custom split), and a receipt drops the guest from its lines and is split
again like the cost editor does. A legacy paid_by_user_id is cleared as on
account deletion. Account deletion goes through the same re-split. The
confirmation text in all languages and the wiki say what really happens.

Refs #2604

client/src/components/Trips/TripMembersModal.test.tsx

@@ -1,4 +1,4 @@
-// FE-COMP-MEMBERS-001 to FE-COMP-MEMBERS-060
+// FE-COMP-MEMBERS-001 to FE-COMP-MEMBERS-061
 import type { Mock } from 'vitest';
 import { act, render, screen, fireEvent, waitFor } from '../../../tests/helpers/render';
 import userEvent from '@testing-library/user-event';
@@ -989,7 +989,7 @@ describe('TripMembersModal', () => {
 
     const trash = await screen.findByRole('button', { name: 'Remove access' });
     await user.click(trash);
-    expect(screen.getByText('Remove this guest? Their assignments and cost shares will be removed too.')).toBeInTheDocument();
+    expect(screen.getByText('Remove this guest? Their assignments are removed and their share of each expense goes to the others in its split.')).toBeInTheDocument();
     await user.click(screen.getByRole('button', { name: 'Cancel' }));
     expect(deletedId).toBeNull();
 
@@ -1015,6 +1015,23 @@ describe('TripMembersModal', () => {
     expect(screen.getByText('Grandma')).toBeInTheDocument();
   });
 
+  it('FE-COMP-MEMBERS-061: a guest who paid for something is kept, with the reason (#2604)', async () => {
+    const user = userEvent.setup();
+    mockRoster([guestRow]);
+    server.use(http.delete('/api/trips/1/guests/:userId', () => HttpResponse.json({ error: 'refused' }, { status: 409 })));
+    render(<TripMembersModal {...defaultProps} />);
+
+    await user.click(await screen.findByRole('button', { name: 'Remove access' }));
+    await user.click(screen.getByRole('button', { name: 'Delete' }));
+
+    await waitFor(() => expect(addToast).toHaveBeenCalledWith(
+      'This guest paid for an expense or is part of a recorded payment. Change the payer or delete the payment first.',
+      'error',
+      undefined,
+    ));
+    expect(screen.getByText('Grandma')).toBeInTheDocument();
+  });
+
   it('FE-COMP-MEMBERS-053: without member_manage a plain member sees no invite or guest controls', async () => {
     const stranger = buildUser({ id: 99, username: 'stranger' });
     seedStore(useAuthStore, { user: stranger, isAuthenticated: true });

client/src/components/Trips/useTripShare.ts

@@ -279,8 +279,11 @@ export function useTripMembers({ isOpen, tripId, onClose, onMembersChanged }: {
       await loadMembers(true)
       await loadBudgetItems(tripId)
       toast.success(t('members.guestRemoved'))
-    } catch {
-      toast.error(t('members.removeError'))
+    } catch (err: unknown) {
+      // A guest who paid for something or is part of a recorded payment stays (#2604):
+      // deleting them would take that money off the books.
+      const refused = (err as { response?: { status?: number } } | null)?.response?.status === 409
+      toast.error(t(refused ? 'members.guestHasPayments' : 'members.removeError'))
     } finally {
       setRemovingId(null)
     }