feat: improve accessibility and SEO (100/100 Lighthouse score)

Fixes color contrast, canonical URLs, viewport scaling, semantic lists,

and resolves 404 errors for manifest/imgproxy.

messages/de.json

@@ -58,12 +58,14 @@
     }
   },
   "Navigation": {
+    "menu": "Menü",
     "home": "KLZ Cables Startseite",
     "team": "Team",
     "products": "Produkte",
     "blog": "Blog",
     "contact": "Kontakt",
-    "toggleMenu": "Menü umschalten"
+    "toggleMenu": "Menü umschalten",
+    "skipToContent": "Zum Inhalt springen"
   },
   "Footer": {
     "legal": "Rechtliches",