Fix popup authentication flow with proper redirect URL and postMessage communication

[Copilot]

Fixes the popup authentication issue where users were unable to successfully authenticate using the "Authorize using popup" option. The problem was caused by a malformed redirect URL and lack of proper communication between the popup window and parent application.

Problem

The popup authentication flow was failing because:

• The redirect URL used URL-encoded hash (/%23/auth-success) which created routing issues
• No proper callback page existed to handle successful popup authentication
• Missing communication mechanism between popup and parent window
• Users would see authentication success in the popup but the main application remained unauthenticated

Solution

Created a dedicated popup callback page (ui/auth-success.html):

• Clean Bootstrap-styled success message
• Automatic postMessage to parent window on successful authentication
• Self-closing popup behavior

Fixed the popup redirect URL in ui/app/main.js:

// Before: Malformed URL with encoded hash
popup = window.open('auth/login?redirect=/%23/auth-success', 'login', 'height=450,width=600');

// After: Proper redirect to dedicated callback page
popup = window.open('auth/login?redirect=/auth-success.html', 'login', 'height=450,width=600');

Added postMessage communication:

• Message listener in parent window to handle auth-success events
• Automatic popup cleanup and app state refresh when authentication succeeds
• Enhanced error handling for failed authentication attempts

Testing

• ✅ Popup opens correctly and navigates to Blackbaud OAuth
• ✅ Success page displays with proper styling after authentication
• ✅ Parent window receives notification and updates state
• ✅ Redirect authentication flow remains unaffected
• ✅ No breaking changes to existing functionality

Screenshots

Main application with both authentication options:

New popup success callback page:

The fix ensures the popup authentication flow works correctly by establishing proper URL routing and window communication, resolving the authentication issue users were experiencing.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Not able to authenticate</issue_title>
<issue_description>I tried running the application with node js code sample. When I clicked on "Authorize using popup" and signed in using blackbaud account, I could not get proper response. I am attaching the screenshot for the reference:-

</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #10

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