feat(nav): add hamburger menu for mobile navigation (Fix #27)

- Hamburger button with animated X toggle (CSS-only icon)
- Slide-down mobile nav on ≤900px with 44px+ tap targets
- Semi-transparent overlay when menu is open
- Escape key + outside click + link click closes menu
- Auto-close on resize to desktop
- Desktop navigation unchanged
- Pure vanilla JS toggle, no jQuery dependency

js/haus-schleusingen.js

@@ -101,3 +101,48 @@ $(function () {
     $("#formSuccess").fadeIn(400);
   });
 });
+
+// Mobile hamburger menu (vanilla JS)
+(function () {
+  var hamburger = document.querySelector(".nav-hamburger");
+  var nav = document.getElementById("navbar");
+  var overlay = document.querySelector(".nav-mobile-overlay");
+  var links = nav ? nav.querySelectorAll(".nav-links a") : [];
+
+  function toggleMenu() {
+    var isOpen = hamburger.classList.toggle("active");
+    nav.classList.toggle("mobile-open", isOpen);
+  if (overlay) overlay.classList.toggle("active", isOpen);
+    hamburger.setAttribute("aria-expanded", isOpen);
+    document.body.style.overflow = isOpen ? "hidden" : "";
+  }
+
+  function closeMenu() {
+    hamburger.classList.remove("active");
+    nav.classList.remove("mobile-open");
+    if (overlay) overlay.classList.remove("active");
+    hamburger.setAttribute("aria-expanded", "false");
+    document.body.style.overflow = "";
+  }
+
+  if (hamburger) {
+    hamburger.addEventListener("click", toggleMenu);
+  }
+
+  if (overlay) {
+    overlay.addEventListener("click", closeMenu);
+  }
+
+  links.forEach(function (link) {
+    link.addEventListener("click", closeMenu);
+  });
+
+  document.addEventListener("keydown", function (e) {
+    if (e.key === "Escape") closeMenu();
+  });
+
+  // Close on resize to desktop
+  window.addEventListener("resize", function () {
+    if (window.innerWidth > 900) closeMenu();
+  });
+})();