docs(firebase_messaging): fix incorrect HTML example for service worker registration

[harshasiddartha]

Description

Fixed incorrect HTML example in the Flutter Web documentation for Firebase Cloud Messaging service worker registration.

Problem

The documentation showed inline JavaScript inside a <script> tag with a src attribute:

<script src="flutter_bootstrap.js" async>
  if ('serviceWorker' in navigator) {
    // ...
  }
</script>

This is incorrect because browsers ignore inline JavaScript when a script tag has a src attribute, which prevents the service worker registration from running.

Solution

Separated the script tags as follows:

<script src="flutter_bootstrap.js" async></script>

<script>
  if ('serviceWorker' in navigator) {
    window.addEventListener('load', function () {
      navigator.serviceWorker.register('firebase-messaging-sw.js', {
        scope: '/firebase-cloud-messaging-push-scope',
      });
    });
  }
</script>

Related Issue

Fixes #17837

Testing

• ✅ Analyzed code with melos analyze-ci - all checks passed
• ✅ Verified HTML syntax is correct
• ✅ Confirmed the fix matches the issue description

Type of Change

• Documentation fix

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[harshasiddartha]

I've signed the Google CLA. The commit email [email protected] is configured correctly in git and is associated with my GitHub account.

Steps taken:

• ✅ Verified git commit email matches: [email protected]
• ✅ Signed Google CLA at https://cla.developers.google.com/
• ✅ Confirmed email is attached to GitHub account

Clicking the "New Contributors: Update this check" link to refresh the CLA status.

This PR fixes the incorrect HTML example in the documentation where inline JavaScript was incorrectly placed inside a script tag with a src attribute, which browsers ignore.

[SelaseKay]

Hi @harshasiddartha, thanks for the contribution. LGTM!