fix(currency): show and settle foreign transactions in AUD

The transactions page rendered the statement's native amount through a
formatter hardcoded to AUD, so a USD row displayed its USD figure labelled as
dollars while every analytics query counted the converted amount_aud. Same
transaction, two different numbers depending on the page.

getTransactions now returns the statement currency, the amount column shows
amount_aud with the native figure beneath it when the two differ, and the split
and duplicate modals seed from the converted amount (a duplicate becomes a
manual AUD row, so the native figure would be wrong there).

Settlement balances had the same split: getParticipantBalances and the
per-participant balance route summed raw amount while trip totals summed
amount_aud, so a shared foreign expense would net a USD figure against AUD
ones. All three now agree on amount_aud. No change to current balances - every
statement in the database is AUD today - but correct once Wise data lands.

src/app/api/participants/[id]/balance/route.ts

@@ -19,7 +19,7 @@ export async function GET(
 
   const rows = await queryRaw<BalanceRow>(
     `SELECT ts.participant_id, p.name,
-       SUM(t.amount * ts.share_percent / 100)::numeric(12,2) as total_owed,
+       SUM(COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100)::numeric(12,2) as total_owed,
        COUNT(*)::int as transaction_count
      FROM transaction_splits ts
      JOIN transactions t ON t.id = ts.transaction_id

src/app/transactions/page.tsx

@@ -22,10 +22,13 @@ function formatDate(d: string) {
 
 const SPEND_TYPES = new Set(["debit", "fee", "interest"]);
 
-function formatAmount(amount: number, type: string) {
+// `amount` is in the statement's native currency; pass the currency to label it
+// correctly. Callers showing a headline figure should pass amount_aud, which is
+// what every analytics query totals.
+function formatAmount(amount: number, type: string, currency = "AUD") {
   const formatted = new Intl.NumberFormat("en-AU", {
     style: "currency",
-    currency: "AUD",
+    currency,
   }).format(amount);
   return SPEND_TYPES.has(type) ? formatted : `+${formatted}`;
 }
@@ -947,7 +950,12 @@ function TransactionsContent() {
                   <td className={`p-2 text-right whitespace-nowrap font-mono ${
                     SPEND_TYPES.has(t.transaction_type) ? "text-red-400" : "text-green-400"
                   }`}>
-                    {formatAmount(t.amount, t.transaction_type)}
+                    {formatAmount(t.amount_aud ?? t.amount, t.transaction_type)}
+                    {t.currency && t.currency !== "AUD" && (
+                      <div className="text-[10px] text-zinc-500 mt-0.5">
+                        {formatAmount(t.amount, t.transaction_type, t.currency)}
+                      </div>
+                    )}
                   </td>
                   <td className="p-2">
                     <EditableTypeBadge
@@ -1000,7 +1008,7 @@ function TransactionsContent() {
                         </span>
                       ))}
                       <button
-                        onClick={() => setSplitModal({ transactionId: t.id, amount: t.amount, description: t.description, merchant: t.effective_merchant || undefined, transactionIds: undefined })}
+                        onClick={() => setSplitModal({ transactionId: t.id, amount: t.amount_aud ?? t.amount, description: t.description, merchant: t.effective_merchant || undefined, transactionIds: undefined })}
                         className={`text-xs px-2 py-0.5 rounded transition-colors ${
                           t.splits?.some((s) => s.participant_id !== me?.id)
                             ? "text-amber-400 hover:text-amber-200 hover:bg-zinc-800"
@@ -1033,7 +1041,9 @@ function TransactionsContent() {
                         prefill: {
                           date: new Date().toISOString().slice(0, 10),
                           description: t.description,
-                          amount: t.amount,
+                          // Duplicates become manual (AUD) transactions, so seed
+                          // them with the converted figure, not the native one.
+                          amount: t.amount_aud ?? t.amount,
                           transaction_type: t.transaction_type,
                           merchant_normalized: t.effective_merchant || undefined,
                           category: t.effective_category || undefined,