fix: cursor border conflicts with grid edges (Issue 2)

Problem: Cursor's border was conflicting with 3x3 box separators
- Cells already have borders for grid structure
- Cursor border overlapped/clipped at edges

Solution: Use outline instead of border
- outline: 3px solid (sits above borders)
- outline-offset: -3px (draws inside cell)
- z-index: 10 + position: relative (layers above grid)

Benefits:
- Outline doesn't affect layout
- Doesn't conflict with existing borders
- Clean appearance at all grid edges
- Consistent thickness throughout

src/game.css

@@ -258,5 +258,8 @@
 /* Cursor highlight */
 .sudoku-cell.cursor {
   background-color: rgba(59, 130, 246, 0.3);
-  border: 2px solid rgb(59, 130, 246);
+  outline: 3px solid rgb(59, 130, 246);
+  outline-offset: -3px;
+  z-index: 10;
+  position: relative;
 }