fix: use DXpedition entity coordinates for active DXpedition spots

[accius]

DXpedition callsigns like TX5EU (Austral Islands) were showing at the generic prefix location (TX → France) because CTY.DAT maps TX to Metropolitan France.

Now cross-references DX cluster spots against the active DXpedition list from NG3K. If a spotted callsign matches a known active DXpedition, uses the DXpedition's DXCC entity coordinates from cty.dat instead of the generic prefix. This runs after grid square resolution but before HamQTH/prefix fallbacks.

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)