feat: add easy, medium, and hard difficulty modes with ghost speed scaling

[Shubhankar-P]

Summary

Adds a difficulty selection screen between the title screen and gameplay.
Players choose Easy, Medium, or Hard before the game starts, which adjusts
the game tick rate and therefore ghost effective speed.

Changes

• Added showDifficultySelect boolean for difficulty screen state
• Added ghostSpeedMultiplier field (1/2/3 for easy/medium/hard)
• New applyDifficulty() method sets timer delay: 70ms / 50ms / 30ms
• Difficulty selection screen rendered in draw() — consistent overlay style
• Title screen now transitions to difficulty screen instead of game directly
• Keys 1/2/3 select difficulty and start the game
• Difficulty re-applied on game restart so chosen mode persists
• actionPerformed() skips move() while on difficulty screen

How to Test

1. Launch game → press any key → difficulty screen appears
2. Press 1 → game starts slow (Easy)
3. Press 2 → game starts at original speed (Medium)
4. Press 3 → game starts fast (Hard)
5. Die and restart → same difficulty applies again

Closes

Closes #7