feat: add bottom navigation bar (mobile-first)

Fixed bottom nav with Home/Spiele/Rangliste/Profil tabs.
Desktop keeps header nav. Admin hidden behind gear icon.
Main content padded to avoid overlap with bottom nav.

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

frontend/src/App.module.css

@@ -96,5 +96,32 @@
   max-width: 1100px;
   margin: 0 auto;
   padding: 32px 24px;
+  padding-bottom: 70px;
   width: 100%;
 }
+
+@media (min-width: 768px) {
+  .main {
+    padding-bottom: 32px;
+  }
+}
+
+/* Hide header nav on mobile */
+@media (max-width: 767px) {
+  .nav {
+    display: none;
+  }
+}
+
+/* Admin link: icon only, subtle */
+.adminLink {
+  display: flex;
+  align-items: center;
+  padding: 4px;
+  color: var(--text-muted);
+  text-decoration: none;
+  transition: color 0.2s;
+}
+.adminLink:hover {
+  color: var(--text-secondary);
+}