feat: Sicherheits-Header fuer das Frontend ergaenzt

apicore.encry.de lieferte bisher gar keine Security-Header aus (per
curl bestaetigt) - CSP, X-Frame-Options, X-Content-Type-Options,
Referrer-Policy, HSTS fehlten komplett, waehrend das Backend das
laengst per helmet() macht. Ergaenzt fuer alle Routen. Bewusst ohne
CSP-Nonce (haette dynamic rendering fuer jede Seite erzwungen und
damit die bestehende statische Optimierung gekostet) - stattdessen
script-src/style-src 'unsafe-inline', aber default-src/connect-src/
img-src/object-src/frame-ancestors/form-action/base-uri strikt
eingeschraenkt. Lokal mit dem echten Production-Build (standalone
server + next.config.ts) verifiziert: Login-Seite rendert/hydratisiert
sauber, keine CSP-Verstoesse in der Konsole, Header per curl bestaetigt.

ApiCore-Frontend/next.config.ts

@@ -1,5 +1,44 @@
 import type { NextConfig } from "next";
 
+// Runs when the Node server process starts inside the container, so this
+// correctly reads the deployment's actual runtime API_URL (see
+// src/lib/config/runtime-config.ts for why API_URL is a runtime env var
+// rather than baked into the client bundle at build time) - this is server
+// startup, not the browser bundle, so it isn't subject to that constraint.
+function apiOrigin(): string {
+  try {
+    return new URL(process.env.API_URL ?? "http://localhost:3001").origin;
+  } catch {
+    return "";
+  }
+}
+
+// No CSP nonce here on purpose: nonce-based CSP requires every page to be
+// dynamically rendered (Next.js CSP guide, "Static vs Dynamic Rendering"),
+// which would drop this app's static optimization across the board. This
+// policy still blocks the things that matter for a mostly-same-origin app -
+// cross-origin exfiltration, clickjacking, base-tag/form hijacking - via
+// 'unsafe-inline' only for script/style (needed for Next's own hydration
+// data and Tailwind's inline styles), matching the same trade-off the
+// backend's own helmet CSP already makes for style-src.
+function cspHeader(): string {
+  return `
+    default-src 'self';
+    script-src 'self' 'unsafe-inline';
+    style-src 'self' 'unsafe-inline';
+    img-src 'self' blob: data:;
+    font-src 'self';
+    connect-src 'self' ${apiOrigin()};
+    object-src 'none';
+    base-uri 'self';
+    form-action 'self';
+    frame-ancestors 'self';
+    upgrade-insecure-requests;
+  `
+    .replace(/\s{2,}/g, " ")
+    .trim();
+}
+
 const nextConfig: NextConfig = {
   output: "standalone",
   async headers() {
@@ -13,6 +52,16 @@ const nextConfig: NextConfig = {
         source: "/sw.js",
         headers: [{ key: "Cache-Control", value: "no-cache, no-store, must-revalidate" }],
       },
+      {
+        source: "/:path*",
+        headers: [
+          { key: "Content-Security-Policy", value: cspHeader() },
+          { key: "X-Frame-Options", value: "SAMEORIGIN" },
+          { key: "X-Content-Type-Options", value: "nosniff" },
+          { key: "Referrer-Policy", value: "no-referrer" },
+          { key: "Strict-Transport-Security", value: "max-age=31536000; includeSubDomains" },
+        ],
+      },
     ];
   },
 };