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

css/haus-schleusingen.css

@@ -115,6 +115,84 @@ nav.scrolled .nav-links a:hover {
   box-shadow: 0 4px 20px rgb(139 105 20 / 50%);
 }
 
+/* HAMBURGER */
+.nav-hamburger {
+  display: none;
+  width: 44px;
+  height: 44px;
+  background: none;
+  border: none;
+  cursor: pointer;
+  position: relative;
+  z-index: 110;
+  align-items: center;
+  justify-content: center;
+  padding: 0;
+}
+
+.nav-hamburger span,
+.nav-hamburger span::before,
+.nav-hamburger span::after {
+  display: block;
+  width: 22px;
+  height: 2px;
+  background: var(--white);
+  border-radius: 1px;
+  transition:
+    transform 0.3s ease,
+    opacity 0.3s ease,
+    background 0.4s;
+  position: absolute;
+}
+
+.nav-hamburger span::before,
+.nav-hamburger span::after {
+  content: "";
+}
+
+.nav-hamburger span::before {
+  transform: translateY(-7px);
+}
+
+.nav-hamburger span::after {
+  transform: translateY(7px);
+}
+
+nav.scrolled .nav-hamburger span,
+nav.scrolled .nav-hamburger span::before,
+nav.scrolled .nav-hamburger span::after {
+  background: var(--dark);
+}
+
+.nav-hamburger.active span {
+  background: transparent;
+}
+
+.nav-hamburger.active span::before {
+  transform: rotate(45deg);
+  background: var(--dark);
+}
+
+.nav-hamburger.active span::after {
+  transform: rotate(-45deg);
+  background: var(--dark);
+}
+
+.nav-mobile-overlay {
+  display: none;
+  position: fixed;
+  inset: 0;
+  background: rgb(28 26 23 / 50%);
+  z-index: 90;
+  opacity: 0;
+  transition: opacity 0.3s ease;
+}
+
+.nav-mobile-overlay.active {
+  display: block;
+  opacity: 1;
+}
+
 /* HERO */
 .hero {
   position: relative;
@@ -1042,6 +1120,59 @@ footer {
     display: none;
   }
 
+  .nav-hamburger {
+    display: flex;
+  }
+
+  /* Mobile slide-down nav */
+  nav.mobile-open .nav-links {
+    display: flex;
+    flex-direction: column;
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    background: rgb(253 252 250 / 98%);
+    backdrop-filter: blur(12px);
+    padding: 5rem 1.5rem 2rem;
+    gap: 0;
+    z-index: 95;
+    border-bottom: 1px solid var(--warm);
+    animation: slideDown 0.3s ease;
+  }
+
+  nav.mobile-open .nav-links a {
+    color: var(--charcoal);
+    font-size: 1rem;
+    padding: 1rem 0;
+    border-bottom: 1px solid var(--warm);
+    display: block;
+    min-height: 44px;
+    display: flex;
+    align-items: center;
+  }
+
+  nav.mobile-open .nav-hamburger span,
+  nav.mobile-open .nav-hamburger span::before,
+  nav.mobile-open .nav-hamburger span::after {
+    background: var(--dark);
+  }
+
+  nav.mobile-open .nav-hamburger.active span {
+    background: transparent;
+  }
+
+  @keyframes slideDown {
+    from {
+      opacity: 0;
+      transform: translateY(-10px);
+    }
+    to {
+      opacity: 1;
+      transform: translateY(0);
+    }
+  }
+
   .hero-content {
     padding: 0 1.5rem 4rem;
   }