Add WPM (Words Per Minute) Counter to Game Over Screen

[vivinarya]

Problem: There's no indicator of typing speed for players.

Task:

• Track and calculate the average words destroyed per minute (WPM) during the game session.
• Display the final WPM score on the Game Over screen.

Acceptance Criteria:

• The WPM counter is visible and accurate on Game Over.
• Calculation is based on actual words destroyed and elapsed playtime.
• Implementation is in script.js only.