Calculate Maidenhead 4 Character Grid

[skyelaird]

Implements the correct approach for VOACAP propagation visualization:

• Uses Maidenhead 4-character grid squares (e.g., FN20, IO91)
• Calculates propagation independently for each grid center
• ~3,240 grid squares covering the globe (1° lat × 2° lon)
• Colors each grid based on likelihood of communications
• Independent of geographic/political boundaries

This approach is scientifically accurate because:

• Radio propagation follows physics, not geography
• Each grid's propagation is calculated separately from TX to RX
• Different grids within the same region can have different propagation
• Maidenhead locator system is the amateur radio standard

Previous mockups (V1-V3) incorrectly tied propagation to geographic regions, which doesn't match how VOACAP calculations work.

Updated MOCKUPS_README.md to:

• Document V4 as the recommended production approach
• Explain why grid-based is superior to region-based visualization
• Provide implementation guidance for production
• Deprecate V1-V3 approaches