Add N-Queens puzzle

Create an N-Queens puzzle where users can place queens on an n x n chessboard. The size of the board can be adjusted using a slider. A timer can be started before placing any queens. Clicking a square places a queen, and clicking a queen removes it. Attacked squares are visually indicated.

src/pages/theme-pages/Puzzles.tsx

@@ -45,6 +45,16 @@ const Puzzles = () => {
       difficulty: "Intermediate" as const,
       duration: "5-15 min",
       participants: "1 player"
+    },
+    {
+      id: "n-queens",
+      title: "N-Queens Puzzle",
+      description: "Place as many queens as possible on an n×n chessboard so that no two queens attack each other.",
+      path: "/puzzles/n-queens",
+      tags: ["Logic", "Chess", "Placement", "Classic"],
+      difficulty: "Intermediate" as const,
+      duration: "5-15 min",
+      participants: "1 player"
     }
   ];