fix(auth): [PM-15987] handle browser back button on login screen

Intercepts browser back button press on the login screen to properly 
transition back to email entry portion instead of unexpected navigation.

Resolves PM-15987

libs/auth/src/angular/login/login.component.ts

@@ -137,6 +137,9 @@ export class LoginComponent implements OnInit, OnDestroy {
   }
 
   async ngOnInit(): Promise<void> {
+    // Add popstate listener to listen for browser back button clicks
+    window.addEventListener("popstate", this.handlePopState);
+
     // TODO: remove this when the UnauthenticatedExtensionUIRefresh feature flag is removed.
     this.listenForUnauthUiRefreshFlagChanges();
 
@@ -148,6 +151,9 @@ export class LoginComponent implements OnInit, OnDestroy {
   }
 
   ngOnDestroy(): void {
+    // Remove popstate listener
+    window.removeEventListener("popstate", this.handlePopState);
+
     if (this.clientType === ClientType.Desktop) {
       // TODO: refactor to not use deprecated broadcaster service.
       this.broadcasterService.unsubscribe(BroadcasterSubscriptionId);
@@ -562,4 +568,28 @@ export class LoginComponent implements OnInit, OnDestroy {
       this.clientType !== ClientType.Browser
     );
   }
+
+  /**
+   * Handle the back button click to transition back to the email entry state.
+   */
+  protected async backButtonClicked() {
+    // Replace the history so the "forward" button doesn't show (which wouldn't do anything)
+    history.pushState(null, "", window.location.pathname);
+    await this.toggleLoginUiState(LoginUiState.EMAIL_ENTRY);
+  }
+
+  /**
+   * Handle the popstate event to transition back to the email entry state when the back button is clicked.
+   * @param event - The popstate event.
+   */
+  private handlePopState = (event: PopStateEvent) => {
+    if (this.loginUiState === LoginUiState.MASTER_PASSWORD_ENTRY) {
+      // Prevent default navigation
+      event.preventDefault();
+      // Replace the history so the "forward" button doesn't show (which wouldn't do anything)
+      history.pushState(null, "", window.location.pathname);
+      // Transition back to email entry state
+      void this.toggleLoginUiState(LoginUiState.EMAIL_ENTRY);
+    }
+  };
 }