feat: add addTransaction function

- Add addTransaction callback to hook
- Use useCallback for performance
- Save to AsyncStorage on add

hooks/useTransactions.ts

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useCallback } from 'react';
 import AsyncStorage from '@react-native-async-storage/async-storage';
 import { Transaction } from '../types';
 
@@ -8,7 +8,7 @@ export const useTransactions = () => {
   const [transactions, setTransactions] = useState<Transaction[]>([]);
   const [loading, setLoading] = useState(true);
 
-  const loadTransactions = async () => {
+  const loadTransactions = useCallback(async () => {
     try {
       setLoading(true);
       const stored = await AsyncStorage.getItem(STORAGE_KEY);
@@ -20,15 +20,24 @@ export const useTransactions = () => {
     } finally {
       setLoading(false);
     }
-  };
+  }, []);
 
   useEffect(() => {
     loadTransactions();
+  }, [loadTransactions]);
+
+  const addTransaction = useCallback((transaction: Transaction) => {
+    setTransactions((prev) => {
+      const updated = [transaction, ...prev];
+      AsyncStorage.setItem(STORAGE_KEY, JSON.stringify(updated));
+      return updated;
+    });
   }, []);
 
   return {
     transactions,
     loading,
+    addTransaction,
     refresh: loadTransactions,
   };
 };