Add features, difficulty scaling, and code quality improvements

[TheSpyHead]

Summary

• High score: Saved and loaded from highscore.txt, displayed in a new panel on the right side. Updates live whenever the current score beats it.
• Pause: Press P to pause/unpause at any time during a game.
• Difficulty scaling: The game timer decreases by 10ms for every 500 points scored, bottoming out at 100ms (from the original fixed 200ms).
• Constants: All magic numbers extracted to a new constants.py — screen size, offsets, timer values, panel positions, block IDs.
• Rendering separation: Next-block preview logic moved from game.py into main.py where it belongs.
• Crash fix: Sound file loading is now wrapped in try/except — the game starts normally even if .ogg files are missing.
• Code cleanup: Replaced == False/== True comparisons with idiomatic not/truthiness checks; fixed mixed tab/space indentation in blocks.py.

Test plan

• Core Gameplay: Start the game and verify normal play works.
• Score Tracking: Score points and confirm the score display updates.
• Persistence: Beat a previous high score and restart — confirm it persists after restart.
• Pause Toggle: Press P mid-game and confirm blocks stop moving; press again to resume.
• Scaling: Score 500+ points and confirm the pieces fall noticeably faster.
• Error Handling: Remove a sound file temporarily and confirm the game still launches.
• Edge Case: Delete highscore.txt and confirm the game creates a new one without crashing.