Fix dead "Katalog" public nav link (Task #50)

Problem: The "Katalog" nav link pointed to "/" — the same page the
visitor is usually on — so it appeared to do nothing, while the actual
"Skill-Katalog" grid sits at the bottom beneath the educational section.

Changes:
- catalog.tsx: gave the "Skill-Katalog" <section> a stable anchor
  (id="skill-katalog") plus scroll-mt-24 to clear the sticky header.
- public-layout.tsx: replaced the generic NAV map with explicit nav
  buttons. The "Katalog" link now has an onClick handler that:
  - smooth-scrolls to #skill-katalog when already on "/",
  - navigates to "/" then scrolls when on another public page.
  A scrollToCatalog() helper uses requestAnimationFrame polling (up to
  20 frames) so the scroll waits until the catalog section has mounted
  after navigation. Active-state highlight for "/" is preserved.

No changes to skill cards, search/filter, or educational content.
Typecheck passes; verified the nav renders with "Katalog" active.

Replit-Task-Id: c1ed232e-e513-4cb8-9079-d239f5d5030c

artifacts/skillguard/src/components/public-layout.tsx

@@ -3,13 +3,29 @@ import { Shield, Search, ShieldCheck, Settings, LayoutDashboard } from "lucide-r
 import { Button } from "@/components/ui/button";
 import { cn } from "@/lib/utils";
 
-const NAV = [
-  { href: "/", label: "Katalog", match: (l: string) => l === "/" },
-  { href: "/pruefen", label: "Skill prüfen", match: (l: string) => l.startsWith("/pruefen") },
-];
+const CATALOG_ANCHOR_ID = "skill-katalog";
+
+function scrollToCatalog(attempts = 20) {
+  const el = document.getElementById(CATALOG_ANCHOR_ID);
+  if (el) {
+    el.scrollIntoView({ behavior: "smooth", block: "start" });
+  } else if (attempts > 0) {
+    requestAnimationFrame(() => scrollToCatalog(attempts - 1));
+  }
+}
 
 export function PublicLayout({ children }: { children: React.ReactNode }) {
-  const [location] = useLocation();
+  const [location, setLocation] = useLocation();
+
+  const handleCatalogClick = (e: React.MouseEvent) => {
+    e.preventDefault();
+    if (location === "/") {
+      scrollToCatalog();
+    } else {
+      setLocation("/");
+      scrollToCatalog();
+    }
+  };
 
   return (
     <div className="flex min-h-screen flex-col bg-background text-foreground">
@@ -20,18 +36,22 @@ export function PublicLayout({ children }: { children: React.ReactNode }) {
             <span className="text-lg font-bold tracking-tight">SkillGuard</span>
           </Link>
           <nav className="flex items-center gap-2">
-            {NAV.map((item) => (
-              <Button
-                key={item.href}
-                asChild
-                variant={item.match(location) ? "secondary" : "ghost"}
-                size="sm"
-              >
-                <Link href={item.href}>
-                  {item.label}
-                </Link>
-              </Button>
-            ))}
+            <Button
+              asChild
+              variant={location === "/" ? "secondary" : "ghost"}
+              size="sm"
+            >
+              <Link href="/" onClick={handleCatalogClick}>
+                Katalog
+              </Link>
+            </Button>
+            <Button
+              asChild
+              variant={location.startsWith("/pruefen") ? "secondary" : "ghost"}
+              size="sm"
+            >
+              <Link href="/pruefen">Skill prüfen</Link>
+            </Button>
             <Button asChild variant="outline" size="sm" className="ml-1">
               <Link href="/admin">
                 <Settings className="mr-1.5 h-4 w-4" />

artifacts/skillguard/src/pages/catalog.tsx

@@ -59,7 +59,7 @@ export default function Catalog() {
 
       <PublicEducation />
 
-      <section className="space-y-6">
+      <section id="skill-katalog" className="scroll-mt-24 space-y-6">
         <div className="flex flex-col gap-2 sm:flex-row sm:items-end sm:justify-between">
           <div>
             <h2 className="text-2xl font-bold tracking-tight">Skill-Katalog</h2>