Add Level Selection Menu for Better Navigation

## Problem
Players must complete levels sequentially. Cannot jump to specific patterns they want to learn or replay completed levels.

## Proposed Solution
Create a main menu screen showing all 4 levels:
- Visual cards for each level
- Lock/unlock status based on completion
- Brief description of what pattern each teaches
- "Play" button to launch level
- Progress indicators

## Benefits
- Improved user experience and navigation
- Allows targeted learning (jump to specific pattern)
- Shows player progress visually
- Professional game standard feature

## Implementation Approach
- New scene: `main_menu.tscn`
- Save/load completion status with ConfigFile
- Level cards with thumbnails and descriptions
- Unlock levels based on previous completion

I'm willing to implement this!