Fix the board cell UI

[vector1505]

🎨 UI Bug: Board cell highlight is incomplete

Description

When a player selects a Sudoku board cell, only the top and left borders are highlighted. The bottom and right borders remain unchanged, which makes the highlighting look incomplete and inconsistent.

Problem

• Cell highlighting only applies to two sides (top and left).
• This creates a broken visual effect and reduces clarity for the player.
• Players may find it harder to keep track of the selected cell.

Expected Behavior

• On selecting a cell, all four borders (top, right, bottom, left) should be highlighted uniformly.
• The highlight should clearly indicate the currently active/selected cell.

Steps to Reproduce

1. Start a Sudoku game.
2. Click on any cell in the grid.
3. Observe that only the top and left borders are highlighted.

Possible Fix

• Update the CSS styles for the selected cell in ./client/src/components/SudokuMasterApp.jsx.
• Ensure that the cell’s highlight class applies border styling to all four sides.
• Optionally, consider using a background color change or a full outline for better visibility.

Additional Context

• Might be caused by border collapsing rules in the CSS grid/table.
• Consider testing with outline, box-shadow, or border applied consistently.