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/pages/MatchesPage.module.css

@@ -1,4 +1,10 @@
-.page { display: flex; flex-direction: column; gap: 16px; }
+.page {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+  max-width: 1100px;
+  margin: 0 auto;
+}
 
 /* ── Stats Card (glassmorphism) ── */
 .statsCard {
@@ -122,6 +128,15 @@
   margin-bottom: 8px;
 }
 
+/* Desktop: 2-column grid for match cards */
+@media (min-width: 768px) {
+  .matchList {
+    display: grid;
+    grid-template-columns: 1fr 1fr;
+    gap: 12px;
+  }
+}
+
 /* ── Past matches toggle ── */
 .pastToggle {
   display: flex;