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/components/BottomNav.tsx

@@ -0,0 +1,29 @@
+import { NavLink } from 'react-router-dom';
+import { Home, Trophy, User } from 'lucide-react';
+import styles from './BottomNav.module.css';
+
+export default function BottomNav() {
+  const linkClass = ({ isActive }: { isActive: boolean }) =>
+    isActive ? styles.tabActive : styles.tab;
+
+  return (
+    <nav className={styles.bottomNav}>
+      <NavLink to="/" end className={linkClass}>
+        <Home size={20} />
+        <span>Home</span>
+      </NavLink>
+      <NavLink to="/spiele" className={linkClass}>
+        <span className={styles.emojiIcon}>⚽</span>
+        <span>Spiele</span>
+      </NavLink>
+      <NavLink to="/rangliste" className={linkClass}>
+        <Trophy size={20} />
+        <span>Rangliste</span>
+      </NavLink>
+      <NavLink to="/profil" className={linkClass}>
+        <User size={20} />
+        <span>Profil</span>
+      </NavLink>
+    </nav>
+  );
+}