[Task]: Refactor `getTargetUrl()` to handle GitHub path conversion

Describe the feature you want to add

Refactor the GitHub path conversion logic so that getTargetUrl() becomes the single reusable function responsible for validating the parsed GitHub username and generating the corresponding GitHub Pages URL.

Describe your proposed solution

Update getTargetUrl() so that it validates the username before generating the target URL.

For example:

function getTargetUrl(username, repositoryName) {
  if (!username || !isValidUserName(username)) {
    return null;
  }
  return repositoryName
    ? `https://${username}.${CONFIG.URLS.PAGES}/${repositoryName}`
    : `https://${username}.${CONFIG.URLS.PAGES}`;
}

Then simplify submitPath() to use the return value from getTargetUrl():

function submitPath(path) {
  const { username, repositoryName } = parseGithubPath(path);
  const targetUrl = getTargetUrl(username, repositoryName);

  if (!targetUrl) {
    return false;
  }

  saveRecent(targetUrl);
  redirectToGithubPages(targetUrl);
  return true;
}

This keeps URL generation and username validation together, without introducing another helper function solely for obtaining the target URL.

Describe alternatives you've considered, if relevant

No response

Additional context

No response

Locate getTargetUrl() and submitPath(), then read the existing parseGithubPath() and isValidUserName() logic they use. Centralize username validation and GitHub Pages URL generation in getTargetUrl(), simplify submitPath() to use its result, and verify valid usernames, repository paths, and invalid or missing usernames retain the expected behavior.