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/tests/unit/mobile/trip/MCostsTab.test.tsx

@@ -626,4 +626,44 @@ describe('MCostsTab', () => {
     expect(del).toHaveBeenCalledWith(7, 502)
     await waitFor(() => expect(settlementBases).toHaveLength(2))
   })
+
+  it('FE-MOB-COSTT-043: editing a payment keeps the currency it was recorded in, not the display currency', async () => {
+    // Recorded in pounds while the phone shows dollars: the row says so, the
+    // sheet reopens in GBP, and a date-only correction must not re-declare the
+    // amount as dollars (the server would freeze a fresh FX rate for it).
+    const payment = { id: 505, from_user_id: 1, to_user_id: 2, amount: 10, currency: 'GBP', settled_at: '2026-04-28', created_at: '2026-04-30T09:00:00Z' }
+    serveSettlement({ ...SETTLEMENT, settlements: [payment] })
+    const update = vi.spyOn(budgetApi, 'updateSettlement').mockResolvedValue({})
+    await renderTab()
+
+    const row = rowOf('costs.payment')
+    expect(within(row).getByText('£10.00 → $20.00')).toBeInTheDocument()
+    expect(within(row).getByText('$20.00')).toBeInTheDocument()
+
+    fireEvent.click(within(row).getByRole('button', { name: 'common.edit' }))
+    const dialog = screen.getByRole('dialog', { name: 'costs.editPayment' })
+    expect(within(dialog).getByDisplayValue('10.00')).toBeInTheDocument()
+    expect(within(dialog).getByRole('button', { name: /GBP/ })).toBeInTheDocument()
+
+    fireEvent.click(within(dialog).getByRole('button', { name: 'common.save' }))
+    expect(update).toHaveBeenCalledWith(7, 505, { from_user_id: 1, to_user_id: 2, amount: 10, currency: 'GBP', settled_at: '2026-04-28' })
+  })
+
+  it('FE-MOB-COSTT-044: a payment cannot be saved without a day', async () => {
+    // Cleared, the server would store NULL and the ledger would quietly fall
+    // back to the day the payment was recorded on.
+    const create = vi.spyOn(budgetApi, 'createSettlement').mockResolvedValue({})
+    await renderTab()
+    fireEvent.click(screen.getByRole('button', { name: 'costs.addPayment' }))
+    const dialog = screen.getByRole('dialog', { name: 'costs.addPayment' })
+    const submit = within(dialog).getByRole('button', { name: 'costs.addPayment' })
+    fireEvent.change(within(dialog).getByPlaceholderText('0.00'), { target: { value: '12,5' } })
+    expect(submit).toBeEnabled()
+
+    fireEvent.click(dialog.querySelector('button[aria-haspopup="dialog"]') as HTMLElement)
+    fireEvent.click(screen.getByRole('button', { name: 'Clear date' }))
+    expect(submit).toBeDisabled()
+    fireEvent.click(submit)
+    expect(create).not.toHaveBeenCalled()
+  })
 })