Fix form submission handler regression from class-to-functional conversion

[longsleep]

Summary

This PR fixes a regression introduced during the conversion of the Login component from class-based to functional component in October 2021.

🐛 Regression Details

Introduced in: d12fad6bd4ab50bdacfae76e7c6631d845c2fcdc (Oct 12, 2021)
Commit: "Add support for visual branding of identifier"
File: identifier/src/containers/Login/Login.jsx:137

What Happened

During the class-to-functional component conversion:

• ✅ The logon method was correctly renamed to handleNextClick
• ✅ The submit button's onClick was correctly updated to use handleNextClick
• ❌ The form's onSubmit was left with the old this.logon(event) reference

Why It Wasn't Noticed

The application continued to work normally because:

1. Enter key presses in form fields trigger the submit button's onClick handler (standard HTML behavior)
2. The onClick={handleNextClick} worked correctly
3. The broken onSubmit handler was never actually called due to browser behavior
4. No console errors were visible to end users

🔧 Fix Applied

Before:

<form action="" onSubmit={(event) => this.logon(event)}>
  // ...
  <Button onClick={handleNextClick}>Next</Button>
</form>

After:

<form action="" onSubmit={handleNextClick}>
  // ...  
  <Button onClick={handleNextClick}>Next</Button>
</form>

✅ Verification

• Form submission now uses the same handler as button click
• Enter key press behavior remains unchanged (works correctly)
• Button click behavior remains unchanged (works correctly)
• No functional regressions introduced
• Code maintainability improved (consistent handler usage)

📋 Testing

Before Fix

• ❌ Form's onSubmit referenced non-existent this.logon method
• ✅ Application worked due to HTML form behavior masking the issue
• ⚠️ Potential confusion for developers debugging form behavior

After Fix

• ✅ Form's onSubmit correctly references handleNextClick
• ✅ Consistent behavior between form submission and button click
• ✅ Clear code path for debugging and maintenance

🎯 Impact

• Risk Level: LOW (fixes broken code, no functional changes)
• User Impact: None (application already worked correctly)
• Developer Impact: Improved code clarity and maintainability
• Regression Age: ~3 years (since Oct 2021)

This is a code quality fix that resolves a long-standing regression without changing any user-visible behavior.