feat(bf): gate the Google Ads tag behind a consent banner; fire the buy-click conversion proxy

/odoo-black-friday/ only, page-side. No Odoo change, no ad-account change.

- static/ads-consent.js (new): default-DENY consent; nothing loads and no request
  fires until Accept. On Decline (or switching to Decline later) the _gcl_au/_gcl_aw
  cookies are cleared. A footer "Cookie settings" link reopens the banner.
- conversion: on submit of either /shop/post_basket buy form (offer block + On the
  Rise card), gtag('event','conversion',{send_to: AW-18065522009/vnDUCPDxt7AdENn6p6ZD})
  then form.submit() from event_callback, with a 700 ms hard-timeout fallback so a
  blocked tag can never swallow the buy. No consent / no tag -> native POST untouched.
- privacy.html: new "Advertising cookies" disclosure (what, why, how to decline).
- Plausible Buy marks and the checkout-side Purchase event untouched.

static/ads-consent.js

@@ -0,0 +1,165 @@
+/* ads-consent.js — Google Ads tag, gated behind a consent banner. Page-side only.
+ *
+ * Page: /odoo-black-friday/. Companion to /static/black-friday.js (which is untouched).
+ *
+ * WHY A GATE: the Google Ads tag is NOT cookieless — it sets the advertising cookies
+ * _gcl_au / _gcl_aw. So nothing loads and no request fires until the visitor accepts.
+ * The default is DENY. Declining genuinely prevents the tag from loading.
+ *
+ * WHAT THE CONVERSION IS: a CLICK PROXY, not a sale. It fires when a buy form on this page
+ * is submitted. The real purchase is measured on the checkout host (20.odoo4projects.com)
+ * by odoo-dev. See wiki/concepts/conversion-tracking-contract. Never label it a sale.
+ *
+ * SAFETY: a conversion tag must never swallow a buy. On submit we fire the event and submit
+ * the form from the event callback, with a hard timeout that submits anyway. With JavaScript
+ * off the native POST forms submit untouched (no banner, no conversion, no error).
+ */
+(function () {
+  "use strict";
+
+  var AW_ID = "AW-18065522009";
+  var SEND_TO = "AW-18065522009/vnDUCPDxt7AdENn6p6ZD";
+  var STORE_KEY = "o4p_ads_consent"; // "granted" | "denied"
+  var SUBMIT_FALLBACK_MS = 700;      // hard cap: submit even if the tag never answers
+
+  /* ---- the visitor's stored choice ---- */
+  function readChoice() {
+    try {
+      return window.localStorage.getItem(STORE_KEY);
+    } catch (e) {
+      return null;
+    }
+  }
+  function saveChoice(value) {
+    try {
+      window.localStorage.setItem(STORE_KEY, value);
+    } catch (e) {
+      /* private mode / storage disabled: treat as no choice, banner stays up */
+    }
+  }
+  function isGranted() {
+    return readChoice() === "granted";
+  }
+
+  /* ---- cookie removal (only used to revoke a prior grant) ---- */
+  function clearCookie(name) {
+    var host = window.location.hostname;
+    var bare = host.replace(/^www\./, "");
+    var paths = ["/", "/odoo-black-friday/"];
+    var domains = ["", "; domain=" + host, "; domain=." + bare];
+    for (var i = 0; i < paths.length; i++) {
+      for (var j = 0; j < domains.length; j++) {
+        document.cookie =
+          name + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=" + paths[i] + domains[j];
+      }
+    }
+  }
+  function clearAdsCookies() {
+    clearCookie("_gcl_au");
+    clearCookie("_gcl_aw");
+  }
+
+  /* ---- the tag: loaded ONLY on consent ---- */
+  function loadGtag() {
+    if (window.__o4pAdsLoaded) return;
+    window.__o4pAdsLoaded = true;
+    window.dataLayer = window.dataLayer || [];
+    window.gtag =
+      window.gtag ||
+      function () {
+        window.dataLayer.push(arguments);
+      };
+    window.gtag("js", new Date());
+    window.gtag("config", AW_ID);
+    var s = document.createElement("script");
+    s.async = true;
+    s.src = "https://www.googletagmanager.com/gtag/js?id=" + AW_ID;
+    document.head.appendChild(s);
+  }
+
+  /* ---- the conversion proxy, fired on a buy-form submit ---- */
+  function onSubmit(e) {
+    var form = e.target;
+    if (!form || form.tagName !== "FORM") return;
+    var action = form.getAttribute("action") || "";
+    if (action.indexOf("/shop/post_basket") === -1) return;
+
+    // No consent, or no tag: do nothing, let the native POST go through untouched.
+    if (!isGranted() || typeof window.gtag !== "function") return;
+
+    e.preventDefault();
+    var submitted = false;
+    function submitNow() {
+      if (submitted) return;
+      submitted = true;
+      form.submit(); // bypasses the submit event: no loop, no re-fire
+    }
+    try {
+      window.gtag("event", "conversion", {
+        send_to: SEND_TO,
+        event_callback: submitNow,
+      });
+    } catch (err) {
+      submitNow();
+    }
+    // Hard fallback: a blocked / slow / failed tag must never stop the purchase.
+    window.setTimeout(submitNow, SUBMIT_FALLBACK_MS);
+  }
+
+  /* ---- the banner ---- */
+  function bannerEl() {
+    return document.getElementById("adsConsent");
+  }
+  function showBanner() {
+    var el = bannerEl();
+    if (el) el.hidden = false;
+  }
+  function hideBanner() {
+    var el = bannerEl();
+    if (el) el.hidden = true;
+  }
+  function accept() {
+    saveChoice("granted");
+    loadGtag();
+    hideBanner();
+  }
+  function decline() {
+    saveChoice("denied");
+    clearAdsCookies();
+    hideBanner();
+  }
+
+  function init() {
+    document.addEventListener("submit", onSubmit, false);
+
+    var banner = bannerEl();
+    if (!banner) return;
+
+    var choice = readChoice();
+    if (choice === "granted") {
+      loadGtag(); // returning visitor who already accepted: load straight away
+    } else if (choice !== "denied") {
+      showBanner(); // first visit (or no stored choice): default deny, ask
+    }
+
+    var acceptBtn = document.getElementById("adsConsentAccept");
+    if (acceptBtn) acceptBtn.addEventListener("click", accept);
+    var declineBtn = document.getElementById("adsConsentDecline");
+    if (declineBtn) declineBtn.addEventListener("click", decline);
+
+    // A real way to change the choice later.
+    var opener = document.getElementById("adsConsentOpen");
+    if (opener) {
+      opener.addEventListener("click", function (e) {
+        e.preventDefault();
+        showBanner();
+      });
+    }
+  }
+
+  if (document.readyState === "loading") {
+    document.addEventListener("DOMContentLoaded", init);
+  } else {
+    init();
+  }
+})();