feat: import transactions from a CSV bank statement

Fourth of the four ideas picked from the Maybe/Sure feature audit, and the
last one queued up. A 3-step wizard (account -> column mapping -> preview)
at /transactions/import, parsed entirely client-side (new lib/csv.ts —
delimiter auto-detect, 5 date formats, thousands/decimal-separator-aware
amounts) and submitted through a new all-or-nothing POST /transactions/bulk
endpoint, mirroring backup restore's validate-everything-before-touching-
anything approach.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/hooks/useTransactions.ts

@@ -1,5 +1,6 @@
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
+  bulkCreateTransactions,
   createTransaction,
   deleteTransaction,
   fetchTransactions,
@@ -56,6 +57,14 @@ export function useUpdateTransaction() {
   });
 }
 
+export function useBulkCreateTransactions() {
+  const invalidate = useInvalidateAfterTransactionChange();
+  return useMutation({
+    mutationFn: (items: TransactionInput[]) => bulkCreateTransactions(items),
+    onSuccess: invalidate,
+  });
+}
+
 export function useDeleteTransaction() {
   const invalidate = useInvalidateAfterTransactionChange();
   return useMutation({