feat(image-zoom): add minimal image zoom functionality

- Introduced CSS for image zoom overlay and image styling.
- Implemented JavaScript for handling image zoom interactions, including overlay creation and close functionality.
- Updated configuration to enable image zoom feature in site parameters.
- Added partial for including image zoom assets in the layout.

assets/js/image-zoom.js

@@ -0,0 +1,76 @@
+// Minimal, dependency-free image zoom for Hextra
+// - Activates on images inside `.content`
+// - Close on overlay click or Escape
+// - Opt-out with `data-no-zoom` on <img>
+
+(function () {
+  function ready(fn) {
+    if (document.readyState === "loading") {
+      document.addEventListener("DOMContentLoaded", fn, { once: true });
+    } else {
+      fn();
+    }
+  }
+
+  function createOverlay(src, alt) {
+    const overlay = document.createElement("div");
+    overlay.className = "hextra-zoom-image-overlay";
+    overlay.setAttribute("role", "dialog");
+    overlay.setAttribute("aria-modal", "true");
+
+    const img = document.createElement("img");
+    img.className = "hextra-zoom-image";
+    img.src = src;
+    if (alt) img.alt = alt;
+
+    overlay.appendChild(img);
+
+    function close() {
+      overlay.classList.remove("show");
+      document.documentElement.style.removeProperty("overflow");
+      window.removeEventListener("keydown", onKeyDown, true);
+      overlay.addEventListener(
+        "transitionend",
+        () => overlay.remove(),
+        { once: true }
+      );
+    }
+
+    function onKeyDown(e) {
+      if (e.key === "Escape") close();
+    }
+
+    overlay.addEventListener("click", close, { once: true });
+    window.addEventListener("keydown", onKeyDown, true);
+
+    document.body.appendChild(overlay);
+    // lock scroll
+    document.documentElement.style.overflow = "hidden";
+
+    // trigger fade-in
+    requestAnimationFrame(() => overlay.classList.add("show"));
+  }
+
+  ready(function () {
+    const container = document.querySelector(".content");
+    if (!container) return;
+
+    container.addEventListener(
+      "click",
+      function (e) {
+        const target = e.target;
+        if (!(target instanceof HTMLImageElement)) return;
+        if (target.dataset.noZoom === "" || target.dataset.noZoom === "true") return;
+
+        // avoid following parent links when zooming
+        e.preventDefault();
+        e.stopPropagation();
+
+        const src = target.currentSrc || target.src;
+        if (!src) return;
+        createOverlay(src, target.alt || "");
+      },
+      true
+    );
+  });
+})();