[MNY-232] SDK: Fix ExecutingTxScreen missing back button in TransactionWidget (#8193)

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## PR-Codex overview
This PR focuses on updating the `onBack` functionality in the `TransactionWidget` and `TransactionModal` components, ensuring that the back navigation behavior is more consistent across these components.

### Detailed summary
- In `TransactionWidget.tsx`, `onBack` now sets the screen to `init-ui`.
- In `TransactionModal.tsx`, `onBack` sets the screen to `deposit`.
- In `ExecutingScreen.tsx`, the `onBack` prop is changed from optional to required.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

* **New Features**
  * Added back navigation from the executing transaction screen, allowing users to return to the previous step without closing the modal.
  * Unified back-button behavior across transaction flows for a more consistent experience.
  * Provides clearer control during long-running executions with an explicit back option.
  * Navigation enhancement applies to both bridge and transaction modal flows, improving step-to-step control while keeping transaction processing unchanged.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: MananTank

packages/thirdweb/src/react/web/ui/Bridge/TransactionWidget.tsx

@@ -770,6 +770,9 @@ function TransactionWidgetContent(
   if (screen.id === "execute-tx") {
     return (
       <ExecutingTxScreen
+        onBack={() => {
+          setScreen({ id: "init-ui" });
+        }}
         closeModal={() => {
           setScreen({ id: "init-ui" });
         }}

packages/thirdweb/src/react/web/ui/TransactionButton/ExecutingScreen.tsx

@@ -20,7 +20,7 @@ export function ExecutingTxScreen(props: {
   tx: PreparedTransaction;
   closeModal: () => void;
   onTxSent: (data: WaitForReceiptOptions) => void;
-  onBack?: () => void;
+  onBack: (() => void) | undefined;
   windowAdapter: WindowAdapter;
 }) {
   const sendTxCore = useSendTransaction({

packages/thirdweb/src/react/web/ui/TransactionButton/TransactionModal.tsx

@@ -133,6 +133,9 @@ function DepositAndExecuteTx(props: ModalProps) {
   if (screen === "execute-tx") {
     return (
       <ExecutingTxScreen
+        onBack={() => {
+          setScreen("deposit");
+        }}
         closeModal={props.onClose}
         onTxSent={props.onTxSent}
         tx={props.tx}