Undo grace for transaction deletes (UX-10)

After confirming a delete the row disappears immediately but the API
call waits 5 seconds behind a Deshacer toast action; undo restores the
row without any server round-trip. A failed commit restores the row
and reports the error.

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

frontend/src/components/TransactionList.tsx

@@ -1,4 +1,4 @@
-import { useState, useMemo } from 'react';
+import { useMemo, useRef, useState } from 'react';
 import {
   Plus,
   Search,
@@ -56,34 +56,70 @@ export default function TransactionList({
   const [modalOpen, setModalOpen] = useState(false);
   const [editing, setEditing] = useState<Transaction | null>(null);
   const [deleteId, setDeleteId] = useState<number | null>(null);
-  const [deleting, setDeleting] = useState(false);
+  const [pendingDeletes, setPendingDeletes] = useState<Set<number>>(new Set());
+  const deleteTimers = useRef<Map<number, ReturnType<typeof setTimeout>>>(new Map());
 
   const handleEdit = (tx: Transaction) => {
     setEditing(tx);
     setModalOpen(true);
   };
 
-  const handleDelete = async () => {
-    if (deleteId === null) return;
-    setDeleting(true);
+  const commitDelete = async (id: number) => {
+    deleteTimers.current.delete(id);
     try {
-      await api.delete(`/transactions/${deleteId}`);
-      toast.success('Transacción eliminada');
-      setDeleteId(null);
+      await api.delete(`/transactions/${id}`);
       onRefresh();
     } catch {
       toast.error('No se pudo eliminar la transacción');
-    } finally {
-      setDeleting(false);
+      setPendingDeletes((prev) => {
+        const next = new Set(prev);
+        next.delete(id);
+        return next;
+      });
     }
   };
 
+  // Undo grace: the row disappears immediately, the API call fires after 5s
+  // unless the user clicks Deshacer (UX-10).
+  const handleDelete = () => {
+    if (deleteId === null) return;
+    const id = deleteId;
+    setDeleteId(null);
+    setPendingDeletes((prev) => new Set(prev).add(id));
+    const toastId = toast('Transacción eliminada', {
+      duration: 5000,
+      action: {
+        label: 'Deshacer',
+        onClick: () => {
+          const timer = deleteTimers.current.get(id);
+          if (timer) clearTimeout(timer);
+          deleteTimers.current.delete(id);
+          setPendingDeletes((prev) => {
+            const next = new Set(prev);
+            next.delete(id);
+            return next;
+          });
+        },
+      },
+    });
+    deleteTimers.current.set(
+      id,
+      setTimeout(() => {
+        toast.dismiss(toastId);
+        void commitDelete(id);
+      }, 5000),
+    );
+  };
+
   const columns = useMemo(
     () => getTransactionColumns({ showCategory, showSourceIcon, onEdit: handleEdit, onDelete: (id) => setDeleteId(id), onToggleDeferred }),
     [showCategory, showSourceIcon, onToggleDeferred],
   );
 
-  const empty = transactions.length === 0 && !loading;
+  const visibleTransactions = pendingDeletes.size
+    ? transactions.filter((t) => !pendingDeletes.has(t.id))
+    : transactions;
+  const empty = visibleTransactions.length === 0 && !loading;
 
   return (
     <>
@@ -195,7 +231,7 @@ export default function TransactionList({
         <CardContent className="p-0">
           <DataTable
             columns={columns}
-            data={transactions}
+            data={visibleTransactions}
             pagination
             pageSize={25}
             initialSorting={[{ id: 'date', desc: true }]}
@@ -220,7 +256,6 @@ export default function TransactionList({
           message="This transaction will be permanently deleted. This action cannot be undone."
           onConfirm={handleDelete}
           onCancel={() => setDeleteId(null)}
-          loading={deleting}
         />
       )}
     </>