fix(budget): keep a payment's currency when it is edited on a phone

The mobile payment sheet always sent the display currency, while it seeded
the amount raw in the currency the payment was recorded in. Correcting only
the day of a 1000 JPY payment turned it into 1000 EUR and froze a fresh FX
rate for it. The sheet now carries its own currency, seeded from the
payment (or the display currency for a new one) with the same picker the
expense sheet has, and the payment row shows the recorded amount next to
the converted one when they differ, like the desktop row does.

The day is now required in both payment forms. Cleared, the server stored
NULL and the ledger quietly fell back to the day the payment was recorded.

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

@@ -1308,6 +1308,21 @@ describe('CostsPanel — payment modal', () => {
     await waitFor(() => expect(put).toBeTruthy())
     expect(put!.settled_at).toBe('2025-06-10')
   })
+
+  it('FE-W5COSTS-077: a payment cannot be saved without a day', async () => {
+    // Cleared, the server would store NULL and the ledger would quietly move
+    // the payment back to the day it was recorded on.
+    const user = userEvent.setup()
+    mount([], { settlements: [{ id: 7, from_user_id: 2, to_user_id: 1, amount: 30, settled_at: '2025-06-10', created_at: '2025-06-16 10:00:00' }] })
+
+    await user.click(await screen.findByTitle('Edit'))
+    const save = screen.getByRole('button', { name: 'Save' })
+    expect(save).toBeEnabled()
+
+    await user.click(document.querySelector('button[aria-haspopup="dialog"]') as HTMLElement)
+    await user.click(screen.getByRole('button', { name: 'Clear date' }))
+    expect(save).toBeDisabled()
+  })
 })
 
 describe('CostsPanel — expense modal', () => {

client/src/components/Budget/CostsPanel.tsx

@@ -1022,7 +1022,7 @@ function SettlementModal({ tripId, people, me, editing, currency, onClose, onSav
   const [saving, setSaving] = useState(false)
 
   const amt = Number.parseFloat(amount) || 0
-  const valid = amt > 0 && fromId !== toId
+  const valid = amt > 0 && fromId !== toId && !!day
   const opts = people.map(p => ({ value: String(p.id), label: p.id === me ? t('costs.you') : p.username }))
 
   const save = async () => {