feat(a11y): ARIA labels, focus management, skip-nav, keyboard nav, contrast fix

Accessibility improvements per WCAG 2.1 AA:
- Skip-to-content link (TA-1)
- ARIA landmarks and roles for nav, main, sections, footer (TA-2)
- Accordion keyboard navigation + aria-expanded (TA-3)
- Lightbox focus trap + focus management + dialog role (TA-4)
- Gallery grid items keyboard accessible (TA-5)
- Improved alt texts for all images (TA-6)
- Focus-visible styles for all interactive elements (TA-7)
- Darker --stone color for WCAG AA contrast compliance (TA-8)

Fix #18

css/haus-schleusingen.css

@@ -1,7 +1,55 @@
+/* SKIP LINK */
+.skip-link {
+  position: absolute;
+  left: -9999px;
+  top: 0;
+  background: var(--accent);
+  color: var(--white);
+  padding: 0.75rem 1.5rem;
+  font-size: 0.85rem;
+  font-weight: 500;
+  z-index: 200;
+  text-decoration: none;
+  transition: none;
+}
+
+.skip-link:focus {
+  left: 0;
+  outline: 2px solid var(--white);
+  outline-offset: 2px;
+}
+
+/* FOCUS VISIBLE */
+*:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+}
+
+.lightbox-close:focus-visible {
+  outline: 2px solid var(--white);
+  outline-offset: 2px;
+}
+
+button:focus-visible,
+a:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+}
+
+.grid-item:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+}
+
+.floor-header:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: -2px;
+}
+
 :root {
   --cream: #f5f0e8;
   --warm: #e8dfd0;
-  --stone: #9e9485;
+  --stone: #7a7062;
   --dark: #1c1a17;
   --charcoal: #2e2b26;
   --accent: #8b6914;