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.

prisma/migrations/migration_lock.toml

@@ -0,0 +1,3 @@
+# Please do not edit this file manually
+# It should be added in your version-control system (e.g., Git)
+provider = "postgresql"