feat: add automatic 404 detection, scroll depth tracking, and scroll depth dashboard card

- 404 detection: checks document.title for "404" or "not found", fires custom event, SPA-aware
- Scroll depth: passive scroll listener fires events at 25/50/75/100% thresholds
- ScrollDepth dashboard card: progress bar visualization showing % of visitors reaching each threshold
- Scroll events filtered out of GoalStats to avoid duplication
- Both features on by default, opt-out via data-no-404 / data-no-scroll

public/script.js

@@ -299,6 +299,10 @@
     if (url !== lastUrl) {
       lastUrl = url;
       trackPageview();
+      // * Check for 404 after SPA navigation (deferred so title updates first)
+      setTimeout(check404, 100);
+      // * Reset scroll depth tracking for the new page
+      if (trackScroll) scrollFired = {};
     }
   }
   new MutationObserver(onUrlChange).observe(document, { subtree: true, childList: true });
@@ -308,7 +312,11 @@
   history.replaceState = function() { _replace.apply(this, arguments); onUrlChange(); };
 
   // * Track popstate (browser back/forward)
-  window.addEventListener('popstate', trackPageview);
+  window.addEventListener('popstate', function() {
+    trackPageview();
+    setTimeout(check404, 100);
+    if (trackScroll) scrollFired = {};
+  });
 
   // * Custom events / goals: validate event name (letters, numbers, underscores only; max 64 chars)
   var EVENT_NAME_MAX = 64;
@@ -346,6 +354,62 @@
   window.pulse = window.pulse || {};
   window.pulse.track = trackCustomEvent;
 
+  // * Auto-track 404 error pages (on by default)
+  // * Detects pages where document.title contains "404" or "not found"
+  // * Opt-out: add data-no-404 to the script tag
+  var track404 = !script.hasAttribute('data-no-404');
+  var sent404ForUrl = '';
+
+  function check404() {
+    if (!track404) return;
+    // * Only fire once per URL
+    var currentUrl = location.href;
+    if (sent404ForUrl === currentUrl) return;
+    if (/404|not found/i.test(document.title)) {
+      sent404ForUrl = currentUrl;
+      trackCustomEvent('404');
+    }
+  }
+
+  // * Check on initial load (deferred so SPAs can set title)
+  setTimeout(check404, 0);
+
+  // * Auto-track scroll depth at 25%, 50%, 75%, and 100% (on by default)
+  // * Each threshold fires once per pageview; resets on SPA navigation
+  // * Opt-out: add data-no-scroll to the script tag
+  var trackScroll = !script.hasAttribute('data-no-scroll');
+
+  if (trackScroll) {
+    var scrollThresholds = [25, 50, 75, 100];
+    var scrollFired = {};
+    var scrollTicking = false;
+
+    function checkScroll() {
+      var docHeight = document.documentElement.scrollHeight;
+      var viewHeight = window.innerHeight;
+      // * Page fits in viewport — nothing to scroll
+      if (docHeight <= viewHeight) return;
+      var scrollTop = window.scrollY;
+      var scrollPercent = Math.round((scrollTop + viewHeight) / docHeight * 100);
+
+      for (var i = 0; i < scrollThresholds.length; i++) {
+        var t = scrollThresholds[i];
+        if (!scrollFired[t] && scrollPercent >= t) {
+          scrollFired[t] = true;
+          trackCustomEvent('scroll_' + t);
+        }
+      }
+      scrollTicking = false;
+    }
+
+    window.addEventListener('scroll', function() {
+      if (!scrollTicking) {
+        scrollTicking = true;
+        requestAnimationFrame(checkScroll);
+      }
+    }, { passive: true });
+  }
+
   // * Auto-track outbound link clicks and file downloads (on by default)
   // * Opt-out: add data-no-outbound or data-no-downloads to the script tag
   var trackOutbound = !script.hasAttribute('data-no-outbound');