feat: extract shop from mp/shop — initial libreshop/shop

Source moved verbatim from mp/shop/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/shop on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

utils/trackEvent.ts

@@ -0,0 +1,13 @@
+declare global {
+	interface Window {
+		umami?: {
+			track: (event: string, data?: Record<string, unknown>) => void;
+		};
+	}
+}
+
+export const trackEvent = (event: string, data?: Record<string, unknown>) => {
+	if (import.meta.client && window.umami) {
+		window.umami.track(event, data);
+	}
+};