Add a hot key (/) to toggle the DE and DX labels

[alanhargreaves]

What does this PR do?

Combine two recent pulls to allow for a hot key (/) to toggle the DE and DX labels.

Type of change

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

How to test

1. Hit a / to toggle the labels.
2. Hit a ? to verify that the information appears in the floating panel
3. In Dockable add the KeyBindingsPanel and verify that the information also appears ther

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