feat: add terrain tiles as a 3D backdrop for outdoor drone shows

[vasarhelyi]

This PR is a preliminary proof-of-concept only. Commit prepared with opencode / DeepSeek V4 Flash Free, only for demonstrating the basic capabilities of the 3d-tiles-renderer npm module integrated into Viewer that is using AFrame based on Three.js.

The PR adds three new settings:

2. 3D tiles source selector (disabled / Google Maps / Cesium Ion)
3. access token (needed for both CesiumIon or Google Maps API integration)

If configured properly, both systems (Cesium and GoogleMaps) should render real 3D terrain instead of the virtual scenery.

Open issues:

• this is a proof-of-concept only, code quality is not checked at all! It is AI generated 100% based on iterative prompting
• if show does not include AMSL show origin altitude, it is initialized as zero. It would be nicer to infer it from the terrain tiles or from a model but the suggested methods to get that are so far too complicated (e.g. raycasting etc.) to be part of a first commit
• I managed to get google maps api working last week when I started things but now it throws 403 error with same asset key, reason unknown yet.
• there are some coordinate conversion tools added but there could be better source for those, I am not familiar enough with the codebase to assess
• new settings are also preliminary only, could be optimized on the UI based on what systems we support etc.
• CesiumIon integration does not contain textured buildings layer yet (only OSM buildings rendered gray)
• There is no copyright display yet on the output when using the terrain from any source

[ntamas]

if show does not include AMSL show origin altitude, it is initialized as zero.

The digital elevation model from the Copernicus mission could be useful. But actually, why not use the Google Elevation API if we require a Google API key anyway?