csv import: map a currency column, or foreign rows land as AUD

Deleting the Frollo importer dropped its currency handling and nothing replaced
it: ColumnMapping had no currency column, so applyMapping could never produce
foreign_currency_code and batchInsertCSVTransactions' support for it was
unreachable from the UI. The USD 10,782 salary imported as A$10,782 — about a
third under, sitting in a column of AUD figures looking entirely normal, which
is the same defect the owner spotted in the first place.

An optional Currency column now sets foreign_currency_code and
foreign_currency_amount when the cell is a three-letter code other than AUD, and
leaves amount_aud NULL rather than inventing a rate. That is the shape order
ingestion already uses and what AMOUNT_UNCONVERTED looks for, so the row renders
as its native figure with "no AUD rate" and the statement supplies the real
number when it arrives.

Caught by reading the imported row rather than the import summary: the summary
said 171 inserted and was right about everything it reported.

src/components/csv-import-modal.tsx

@@ -300,12 +300,14 @@ export function CsvImportModal({ onClose }: { onClose: () => void }) {
                 <ColSelect label="Merchant Column (optional)" value={mapping.merchantCol ?? ""} onChange={(v) => setMapping((m) => ({ ...m, merchantCol: v || undefined }))} options={columnLabels} />
                 <ColSelect label="Category Column (optional)" value={mapping.categoryCol ?? ""} onChange={(v) => setMapping((m) => ({ ...m, categoryCol: v || undefined }))} options={columnLabels} />
                 <ColSelect label="Account Column (optional)" value={mapping.accountCol ?? ""} onChange={(v) => setMapping((m) => ({ ...m, accountCol: v || undefined }))} options={columnLabels} />
+                <ColSelect label="Currency Column (optional)" value={mapping.currencyCol ?? ""} onChange={(v) => setMapping((m) => ({ ...m, currencyCol: v || undefined }))} options={columnLabels} />
                 <ColSelect label="Row ID Column (optional)" value={mapping.sourceRefCol ?? ""} onChange={(v) => setMapping((m) => ({ ...m, sourceRefCol: v || undefined }))} options={columnLabels} />
               </div>
               <p className="text-[11px] text-zinc-500 leading-relaxed">
                 Map <b>Account</b> when the file covers more than one account: rows already
                 covered by that account&apos;s statements are then left out. Map <b>Row ID</b> to
-                make re-importing the same file do nothing.
+                make re-importing the same file do nothing. Map <b>Currency</b> for a
+                multi-currency file, or foreign rows are stored as if they were AUD.
               </p>
 
               <label className="flex items-center gap-2 text-sm cursor-pointer text-zinc-400">