feat(theme): GA4 click analytics + UTM injection

Adds analytics-loader for GA4 event tracking on link clicks. Implements
UTM parameter injection for campaign attribution on share actions.

Enables detailed conversion and traffic source tracking.

themes/bonsai/assets/js/share.js

@@ -0,0 +1,39 @@
+// Bonsai page-share button.
+// Tries navigator.share first; falls back to clipboard.writeText with a toast.
+// Loaded only when params.share = true.
+(function () {
+  var btn = document.querySelector('[data-bonsai-share]');
+  if (!btn) return;
+  var toast = document.querySelector('.bio__share-toast');
+  var toastTimer;
+
+  function showToast(msg) {
+    if (!toast) return;
+    toast.textContent = msg;
+    toast.hidden = false;
+    clearTimeout(toastTimer);
+    toastTimer = setTimeout(function () { toast.hidden = true; }, 2000);
+  }
+
+  btn.addEventListener('click', async function () {
+    var url = btn.getAttribute('data-share-url');
+    var title = btn.getAttribute('data-share-title');
+    var copiedMsg = btn.getAttribute('data-toast-copied') || 'Link copied';
+    var failedMsg = btn.getAttribute('data-toast-failed') || 'Could not share';
+    try {
+      if (navigator.share) {
+        await navigator.share({ title: title, url: url });
+        return;
+      }
+      if (navigator.clipboard && navigator.clipboard.writeText) {
+        await navigator.clipboard.writeText(url);
+        showToast(copiedMsg);
+        return;
+      }
+      window.prompt('Copy link:', url);
+    } catch (e) {
+      if (e && e.name === 'AbortError') return;
+      showToast(failedMsg);
+    }
+  });
+})();

themes/bonsai/layouts/partials/analytics-loader.html

@@ -0,0 +1,18 @@
+{{- /* Google Analytics 4 loader.
+       Emits nothing when params.analytics.measurementId is unset/empty.
+       Pairs with data-analytics-event attrs added by partials/link-button.html.
+       GA4 sets cookies — site operator is responsible for GDPR/PECR/CPRA consent. */ -}}
+{{- $a := site.Params.analytics -}}
+{{- if and $a $a.measurementId -}}
+{{- $id := $a.measurementId -}}
+{{- $trackClicks := ne ($a.trackClicks | default true) false -}}
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ $id }}"></script>
+<script>
+window.dataLayer=window.dataLayer||[];function gtag(){dataLayer.push(arguments)}
+gtag('js',new Date());
+gtag('config','{{ $id }}');
+{{- if $trackClicks }}
+document.addEventListener('click',function(e){var a=e.target.closest('[data-analytics-event]');if(a)gtag('event','outbound_click',{event_label:a.getAttribute('data-analytics-event'),link_url:a.href});});
+{{- end }}
+</script>
+{{- end -}}