feat: add standalone Nim game page, route, and container layout; remove global header/footer from /games/nim

src/pages/Index.tsx

@@ -62,6 +62,13 @@ const allInteractives = [{
   tags: ['chess', 'knights', 'puzzle'],
   path: '/knights-puzzle',
   theme: 'Puzzles'
+}, {
+  id: 'nim',
+  title: 'Game of Nim',
+  description: 'A classic two-player game of strategy. Take turns removing stones from heaps. The player to take the last stone wins!',
+  tags: ['strategy', 'two-player', 'math', 'logic'],
+  path: '/games/nim',
+  theme: 'Games'
 }];
 
 // Featured interactives (3 fixed ones)