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