fix: built-in propagation model was severely underestimating MUF

[accius]

The foF2 formula produced ~3 MHz when real ionosondes show 8-12 MHz at SFI=150 daytime. This made every band appear closed/poor.

Rewrote with empirical fit to ionosonde data:

• foF2_day = (4 + 0.04·SFI) — gives 10 MHz at SFI=150 (was ~3)
• foF2_night = (2 + 0.01·SFI) — proper nighttime floor
• Smooth day/night blend peaking at 14:00 local solar time
• M-factor varies with distance (was fixed at 3.0)
• K-index: no penalty below K=4 (K=3 is normal conditions)
• Multi-hop: 5% per hop (was 8%)
• Polar penalty: only above 65° (was 60°)
• LUF base: 2 MHz (was 3 MHz)

Results now match real-world experience:
20m US→EU noon SFI=150: 81% (was ~0%) 15m US→EU noon SFI=150: 98% (was ~0%) 40m US→EU night: 96% 10m solar minimum: closed ✓

What does this PR do?

Type of change

• Bug fix
• New feature
• Performance improvement
• Refactor / code cleanup
• Documentation
• Translation
• Map layer plugin

How to test

Checklist

• App loads without console errors
• Tested in Dark, Light, and Retro themes
• Responsive at different screen sizes (desktop + mobile)
• If touching server.js: caches have TTLs and size caps (we serve 2,000+ concurrent users)
• If adding an API route: includes caching and error handling
• If adding a panel: wired into Modern, Classic, and Dockable layouts
• No hardcoded colors — uses CSS variables (var(--accent-cyan), etc.)
• No .bak, .old, console.log debug lines, or test scripts included

Screenshots (if visual change)