fix(costs): allow recording an expense with no split or payer (#1286)

Adding an expense required at least one participant, so a cost you only
want to record — e.g. a booking paid on-site later — could not be saved
without splitting it. Drop the participant requirement: with nobody
selected the expense saves as a recorded total, counted in the trip
total and shown as Unfinished, and kept out of settlements until
who-paid is filled in. The shared schema and server already supported
this case.

client/src/components/Budget/CostsPanel.test.tsx

@@ -159,4 +159,39 @@ describe('CostsPanel — settlements in the ledger', () => {
     await screen.findByText('Hotel')
     expect(screen.getByText('Unfinished')).toBeInTheDocument()
   })
+
+  it('records a recorded-total expense with nobody to split with (#1286)', async () => {
+    let posted: Record<string, unknown> | null = null
+    server.use(
+      http.get('/api/trips/1/budget', () => HttpResponse.json({ items: [] })),
+      http.get('/api/trips/1/budget/settlement', () => HttpResponse.json({ balances: [], flows: [], settlements: [] })),
+      http.post('/api/trips/1/budget', async ({ request }) => {
+        posted = await request.json() as Record<string, unknown>
+        return HttpResponse.json({ item: { ...buildBudgetItem({ trip_id: 1, name: 'Hotel' }), id: 9 } })
+      }),
+    )
+    const { default: userEvent } = await import('@testing-library/user-event')
+    const user = userEvent.setup()
+    render(<CostsPanel tripId={1} tripMembers={tripMembers} />)
+
+    await user.click(await screen.findByRole('button', { name: 'Add expense' }))
+    await user.type(await screen.findByPlaceholderText('e.g. Dinner, souvenirs, gas…'), 'Hotel')
+    await user.type(screen.getAllByPlaceholderText('0.00')[0], '120') // total only, paid on-site later
+
+    // Deselect everyone — the cost is recorded without a split (the bug: this was blocked).
+    // The participant toggles are buttons; the same names also appear as plain text in
+    // the Balances sidebar, so target the buttons specifically.
+    await user.click(screen.getByRole('button', { name: /alice/i }))
+    await user.click(screen.getByRole('button', { name: /bob/i }))
+
+    const addBtns = screen.getAllByRole('button', { name: 'Add expense' })
+    const submit = addBtns[addBtns.length - 1] // footer submit
+    expect(submit).not.toBeDisabled()
+    await user.click(submit)
+
+    await waitFor(() => expect(posted).toBeTruthy())
+    expect(posted!.total_price).toBe(120)
+    expect(posted!.member_ids).toEqual([])
+    expect(posted!.payers).toEqual([])
+  })
 })

client/src/components/Budget/CostsPanel.tsx

@@ -818,7 +818,10 @@ export function ExpenseModal({ tripId, base, people, me, editing, prefill, onClo
   const paidEntered = paidSum > 0
   const balanced = Math.abs(paidSum - totalNum) < 0.01
   const each = participants.size > 0 ? totalNum / participants.size : 0
-  const valid = name.trim().length > 0 && totalNum > 0 && participants.size > 0 && (!paidEntered || balanced)
+  // No participants = a recorded total with nobody to split with (e.g. a booking
+  // paid on-site later). It saves as an "unfinished" expense (#1286); selecting
+  // people only adds the who-owes-whom split on top.
+  const valid = name.trim().length > 0 && totalNum > 0 && (!paidEntered || balanced)
 
   // Spread `amount` across `n` people in whole cents so the parts sum back exactly.
   const splitCents = (amount: number, n: number): number[] => {
@@ -978,7 +981,7 @@ export function ExpenseModal({ tripId, base, people, me, editing, prefill, onClo
           </div>
           <div style={{ marginTop: 10, fontSize: 12.5, display: 'flex', justifyContent: 'space-between', gap: 10, flexWrap: 'wrap' }}>
             <span className="text-content-faint">
-              {participants.size === 0 ? t('costs.pickSomeone') : t('costs.splitSummary', { count: participants.size, amount: sym(currency) + each.toFixed(2) })}
+              {participants.size > 0 && t('costs.splitSummary', { count: participants.size, amount: sym(currency) + each.toFixed(2) })}
             </span>
             {paidEntered
               ? <span style={{ fontWeight: 600, color: balanced ? '#16a34a' : '#dc2626' }}>{sym(currency)}{paidSum.toFixed(2)} / {sym(currency)}{totalNum.toFixed(2)}</span>