feat: mobile responsive — sidebar bottom nav + grids adaptatifs

- Sidebar → bottom nav fixe sur mobile (<768px)
- Classes CSS layout: .sidebar, .nav-item, .grid-2, .layout-*
- Grids 2col → 1col sur mobile (Dashboard, Combat, Forge)
- HudBar compact + wrapping sur mobile
- GuideDrawer full-width mobile
- Cards padding réduit mobile
- Header username masqué mobile

frontend/src/pages/CombatPage.tsx

@@ -213,7 +213,7 @@ export function CombatPage() {
     <div>
       <h2 style={{ margin: '0 0 1rem', color: '#f4c94e', fontSize: 20 }}>⚔️ Combat</h2>
 
-      <div style={{ display: 'grid', gridTemplateColumns: '1fr 1fr', gap: '1rem', marginBottom: '1rem' }}>
+      <div className="grid-2" style={{ marginBottom: '1rem' }}>
         {/* Choix monstre par zone */}
         <div>
           {Array.from(monstersByZone.entries()).map(([zone, zoneMonsters]) => {