[Fix] Maidenhead grid special case on anti-meridian. Add meridian & anti-meridian unit test cases.

[MichaelWheeley]

What does this PR do?

• This is NOT an urgent PR, it is a fix for a special case identified.
• Strictly {lat:0, lon:180} is not valid since it should correctly be written {lat:0, lon:-180}, these are both points on the anti-meridian with locator AJ00aa.
• Code accepts {lat:0, lon:180} since it might sometimes be expected to be functional. Problem was identified that {lat:0, lon:180} was calculating SJ00aa which is an invalid locator.
• Unit test cases added on meridian and anti-meridian.
• Remove box-coordinate test from Sydney, one test San Diego is sufficient for coverage since the box-coordinate function is also used internally.

Type of change

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

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