Fix: Update button remaining disabled in transaction edit mode

This commit resolves an issue where the Update button in the transaction form
would remain disabled when attempting to edit a transaction. The problem was
in how the transactionStore was managing state updates during transaction editing.

Key changes:
- Enhanced startEditingTransaction function in transactionStore.ts to ensure proper reactivity
- Added clean copy creation of transaction objects to avoid reference issues
- Implemented a state update cycle with null value first to force reactivity
- Added a small timeout to ensure state changes are properly detected by components

The Transaction form now correctly enables the Update button when in edit mode,
regardless of account selection state.

src/components/MainContent.astro

@@ -10,7 +10,7 @@ const { account } = Astro.props;
 ---
 <main class="main-content">
     <header class="main-header">
-         <h1>Transactions for <span id="current-account-name">{account.name} (***{account.last4})</span></h1>
+         <h1>Transactions for <span id="current-account-name">{account.name} (***{account.accountNumber.slice(-3)})</span></h1>
     </header>
     <TransactionTable client:load />
 </main>