feat: add pause menu with P key toggle and resume functionality

[Shubhankar-P]

Summary

Adds a pause menu that allows players to pause and resume the game mid-play.

Changes

• Added paused boolean field to track pause state
• Draw semi-transparent pause overlay in draw() with PAUSED title, resume hint, and restart hint — consistent with existing showInstructions overlay style
• Skip move() in actionPerformed() when paused so ghosts and pacman freeze in place
• Handle P key in keyReleased() to toggle pause
• Handle R key in pause menu to restart the game
• Guard arrow keys when paused to prevent queued direction changes

How to Test

1. Start the game and begin playing
2. Press P — game should freeze, pause overlay appears
3. Press P again — game resumes from exact position
4. Press P, then R — game restarts fresh

Related Issue

Closes #3