Implement banking system features and enhancements

- Added banking mode with checking, savings, and charity accounts, including auto-split options for income.
- Introduced banking transaction management, including transfers and charity outflows.
- Updated family settings to allow configuration of banking features and interest rates.
- Enhanced data export functionality to include bank transactions.
- Improved user interface to display banking information and donation goals.
- Updated documentation to reflect new banking features and settings.

api/data_snapshot.php

@@ -19,6 +19,7 @@ $allowed = [
     'meal_plans' => 'meal_plans.json',
     'family_settings' => 'family_settings.json',
     'expenses' => 'expenses.json',
+    'bank_transactions' => 'bank_transactions.json',
 ];
 
 $type = isset($_GET['type']) ? trim((string) $_GET['type']) : '';