feat: add logging for blocked updates when beforeunload is cancelled

[nakjun12]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

I have added a new example to manually reproduce this bug and verify the logging behavior.

Motivation / Use-Case

This PR adds logging to help developers identify when webpack-dev-server ignores HMR and live reload updates after canceling the beforeunload confirmation dialog.

The Problem:
When users cancel the browser's "Leave site?" dialog, the internal isUnloading state gets stuck as true, silently blocking all subsequent HMR updates.

Common Scenarios:

• Form pages: Users cancel navigation to avoid losing unsaved data
• Development tools: Tools like LocatorJS trigger beforeunload events when opening files in VSCode

Why This Matters:
Most developers don't realize their updates are being blocked because there's no visible indication. This creates a silent workflow disruption that can be frustrating to diagnose.

Solution:
This PR adds informative logging to make the blocking behavior visible, helping developers understand when and why their updates are being ignored.

Fixes #5571

---

Proposed Solutions

This PR improves debugging through logging.
I've also explored several approaches to address the root cause and would welcome maintainer feedback on implementation.

Background: I tried using various browser events and setTimeout approaches, but couldn't find reliable indicators for when unload is cancelled.
These proposals consider the discussion in #841.

Option 1: Configuration Control

// webpack.config.js
module.exports = {
  devServer: {
    beforeunload: false // or 'ignore'
  }
};

Let users disable this feature when it disrupts their workflow.

Option 2: Auto Reset State

Reset isUnloading to false immediately after blocking one reload to prevent permanent blocking.

Option 3: Conditional Listener

Only enable beforeunload listener when liveReload: true, since HMR doesn't need full page reloads.

[alexander-akait]

Why we need this example?

[nakjun12]

I initially added this example while working on the improvements, but I kept it because it clearly demonstrates the problem I was experiencing and helps with understanding the issue.

[alexander-akait]

Do you want to fix the problem, not just logging?

[nakjun12]

Yes, I tried various approaches but concluded that logging would be sufficient for now.
Would you have any suggestions for a better approach?

[alexander-akait]

Unfortunately I don't think it can be fixed here, if you have logic beforeunload you need to do it manually refresh the page

[nakjun12]

I understand.
I think logging alone would help developers understand why HMR stops working.
I'll remove the examples. Would that be okay?

[alexander-akait]

Let's make only one example and provide lines how to fix it (i.e. reloading) - adding comment in code and in readme (in example)