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.tsx

@@ -1,10 +1,11 @@
 import { useState, useEffect } from 'react';
 import { Routes, Route, NavLink } from 'react-router-dom';
-import { Sun, Moon } from 'lucide-react';
+import { Sun, Moon, Settings } from 'lucide-react';
 import MatchesPage from './pages/MatchesPage';
 import LeaderboardPage from './pages/LeaderboardPage';
 import ProfilePage from './pages/ProfilePage';
 import AdminPage from './pages/AdminPage';
+import BottomNav from './components/BottomNav';
 import styles from './App.module.css';
 
 const IS_DEV = import.meta.env.DEV || import.meta.env.VITE_TEST_MODE === 'true';
@@ -79,7 +80,7 @@ export default function App() {
             )}
           </div>
           <nav className={styles.nav}>
-            <NavLink to="/" end className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
+            <NavLink to="/spiele" className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
               Spielplan
             </NavLink>
             <NavLink to="/rangliste" className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
@@ -88,8 +89,8 @@ export default function App() {
             <NavLink to="/profil" className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
               Mein Profil
             </NavLink>
-            <NavLink to="/admin" className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
-              Admin
+            <NavLink to="/admin" className={styles.adminLink} title="Admin">
+              <Settings size={16} />
             </NavLink>
             <button
               className={styles.themeToggle}
@@ -106,12 +107,15 @@ export default function App() {
       <main className={styles.main}>
         <Routes>
           <Route path="/" element={<MatchesPage key={refreshKey} devUser={devUser} />} />
+          <Route path="/spiele" element={<MatchesPage key={refreshKey} devUser={devUser} />} />
           <Route path="/rangliste" element={<LeaderboardPage key={refreshKey} />} />
           <Route path="/profil" element={<ProfilePage key={refreshKey} />} />
           <Route path="/admin" element={<AdminPage />} />
         </Routes>
       </main>
 
+      <BottomNav />
+
       {IS_DEV && DevPanel && (
         <DevPanel
           currentUser={devUser}