feat: move Admin to bottom nav, remove from header

frontend/src/App.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect } from 'react';
 import { Routes, Route, NavLink } from 'react-router-dom';
-import { Sun, Moon, Settings } from 'lucide-react';
+import { Sun, Moon } from 'lucide-react';
 import DashboardPage from './pages/DashboardPage';
 import MatchesPage from './pages/MatchesPage';
 import LeaderboardPage from './pages/LeaderboardPage';
@@ -95,9 +95,6 @@ export default function App() {
             </NavLink>
           </nav>
           <div className={styles.headerActions}>
-            <NavLink to="/admin" className={styles.adminLink} title="Admin">
-              <Settings size={16} />
-            </NavLink>
             <button
               className={styles.themeToggle}
               onClick={toggleTheme}