fix: use local Inter font to avoid Google Fonts timeout in Docker build

The Docker container cannot reach Google Fonts, causing build failures.
Switch to bundled local font file using next/font/local.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin-lehrer/app/layout.tsx

@@ -1,8 +1,12 @@
 import type { Metadata } from 'next'
-import { Inter } from 'next/font/google'
+import localFont from 'next/font/local'
 import './globals.css'
 
-const inter = Inter({ subsets: ['latin'] })
+const inter = localFont({
+  src: '../public/fonts/Inter-VariableFont.woff2',
+  variable: '--font-inter',
+  display: 'swap',
+})
 
 export const metadata: Metadata = {
   title: 'BreakPilot Admin Lehrer KI',