feat: desktop layout optimization

Dashboard: 2-column layout (hero left 60%, stats+nudges right 40%)
Spielplan: 2-column grid for match cards on desktop
Profile: wider max-width (900px), 6-column achievement grid
Header: Admin link back in desktop nav, max-width 1200px
All via CSS media queries (min-width: 768px), no HTML restructuring.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.module.css

@@ -109,7 +109,7 @@
 
 .main {
   flex: 1;
-  max-width: 1100px;
+  max-width: 1200px;
   margin: 0 auto;
   padding: 32px 24px;
   padding-bottom: 70px;