feat(frontend): adapt to composite key transaction structure

- Update Transaction interface to include stable transaction_id field
- Modify TransactionsList to use stable transaction_id for React keys
- Update API models to handle new transactionId field from database
- Fix API routes to properly map transaction_id in responses
- Update test mocks to include transactionId field
- Ensure backward compatibility with internal_transaction_id

This adapts the frontend to work with the new composite primary key
(accountId, transactionId) structure that prevents duplicate transactions.

leggend/api/routes/transactions.py

@@ -75,7 +75,8 @@ async def get_all_transactions(
             # Return simplified transaction summaries
             data = [
                 TransactionSummary(
-                    internal_transaction_id=txn["internalTransactionId"],
+                    transaction_id=txn["transactionId"],  # NEW: stable bank-provided ID
+                    internal_transaction_id=txn.get("internalTransactionId"),
                     date=txn["transactionDate"],
                     description=txn["description"],
                     amount=txn["transactionValue"],
@@ -89,7 +90,8 @@ async def get_all_transactions(
             # Return full transaction details
             data = [
                 Transaction(
-                    internal_transaction_id=txn["internalTransactionId"],
+                    transaction_id=txn["transactionId"],  # NEW: stable bank-provided ID
+                    internal_transaction_id=txn.get("internalTransactionId"),
                     institution_id=txn["institutionId"],
                     iban=txn["iban"],
                     account_id=txn["accountId"],