build: Schriften lokal statt über Google (next/font/local)

Space Grotesk und Inter als variable Fonts, IBM Plex Mono 400/500, je
latin (enthält ä ö ü ß „ “ – €), unverändert aus @fontsource 5.3.0
(OFL-1.1, Hinweis in src/app/fonts/OFL.txt). Der Build holt nichts mehr
von Google; behebt den sporadischen Turbopack-Fehler beim Laden von
next/font/google im CI und spart einen Drittanbieter-Abruf.

Gates: Frontend lint, typecheck, 3185 Tests, build, 3D-Budget.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Frontend/src/app/layout.tsx

@@ -1,25 +1,32 @@
 import type { Metadata, Viewport } from "next";
-import { Space_Grotesk, Inter, IBM_Plex_Mono } from "next/font/google";
+import localFont from "next/font/local";
 import "./globals.css";
 import { Providers } from "./providers";
 import { Toaster } from "@/components/ui/sonner";
 import { ServiceWorkerRegistration } from "@/components/service-worker-registration";
 
-const spaceGrotesk = Space_Grotesk({
+// Self-hosted (OFL-1.1, see ./fonts/OFL.txt): no Google request at build or runtime.
+const spaceGrotesk = localFont({
   variable: "--font-heading",
-  subsets: ["latin"],
-  weight: ["500", "600", "700"],
+  src: "./fonts/space-grotesk-latin-wght-normal.woff2",
+  weight: "300 700",
+  display: "swap",
 });
 
-const inter = Inter({
+const inter = localFont({
   variable: "--font-body",
-  subsets: ["latin"],
+  src: "./fonts/inter-latin-wght-normal.woff2",
+  weight: "100 900",
+  display: "swap",
 });
 
-const plexMono = IBM_Plex_Mono({
+const plexMono = localFont({
   variable: "--font-data",
-  subsets: ["latin"],
-  weight: ["400", "500"],
+  src: [
+    { path: "./fonts/ibm-plex-mono-latin-400-normal.woff2", weight: "400", style: "normal" },
+    { path: "./fonts/ibm-plex-mono-latin-500-normal.woff2", weight: "500", style: "normal" },
+  ],
+  display: "swap",
 });
 
 export const metadata: Metadata = {