feat: collect and track utms at signup

[Zaimwa9]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

The point of this PR is to collect (from the webflow redirections ✅ ) the marketing UTMs. They are passed at signup, saved along Hubspot tracker and submitted within the create_lead_form

• Made hubspot_cookie optional in Hubspot_tracker (could be normalized and a new schema for UTMs)
• Parse, store and provide UTMs when creating lead form

Webflow
Classic parse and store UTMs params code (settings -> Custom Code -> head code

<script>
  // UTM capture
  // --- CONFIG ---
const UTM_KEYS = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content'];
const STORAGE_KEY = 'flagsmith_utm_data';

// --- CAPTURE UTM PARAMETERS FROM URL ---
function getUTMParamsFromURL() {
  const params = new URLSearchParams(window.location.search);
  console.log(params, window.location)
  const utms = {};
  UTM_KEYS.forEach(key => {
    const val = params.get(key);
    if (val) utms[key] = val;
  });
  return utms;
}

// --- SAVE UTM PARAMS TO LOCALSTORAGE ---
function saveUTMToLocalStorage(utms) {
  if (Object.keys(utms).length > 0) {
    localStorage.setItem(STORAGE_KEY, JSON.stringify(utms));
  }
}

// --- GET UTM DATA FROM LOCALSTORAGE ---
function getUTMFromLocalStorage() {
  try {
    return JSON.parse(localStorage.getItem(STORAGE_KEY)) || {};
  } catch {
    return {};
  }
}

// --- RUN ---
const utmParams = getUTMParamsFromURL();
if (Object.keys(utmParams).length > 0) {
  saveUTMToLocalStorage(utmParams);
}
  
// --- Update app redirect links with UTMs ---
document.addEventListener('DOMContentLoaded', () => {
  const utms = JSON.parse(localStorage.getItem('flagsmith_utm_data') || '{}');
  if (Object.keys(utms).length === 0) return;

  const links = document.querySelectorAll('a[href]');
  const matchUrl = /^https:\/\/app\.flagsmith\.com(\/.*)?$/;

  links.forEach((link) => {
    const href = link.getAttribute('href');
    if (!matchUrl.test(href)) return;

    const url = new URL(href);
    Object.entries(utms).forEach(([k, v]) => url.searchParams.set(k, v));
    link.setAttribute('href', url.toString());
  });
});
</script>

How did you test this code?

• Tests

Steps to reproduce

• Activate hubspot lead tracking
• create a user by coming to http://localhost:8080/signup?utm_campaign=xxx&utm_source=yyy[...]
• verify in hubspot they have been tracked

[khvn26]

Just a couple comments (one is a reply to @matthewelwell's thread)

[khvn26]

LGTM 👍