fix(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

[gemini-code-assist]

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[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.