Apply a subtle highlight to structures on mouse hover

Sometimes a right-click on a structure doesn't work on the first attempt

Hovering over your own structure will slightly change its icon's contrast. This confirms that a right-click will hit that exact structure.

This would be helpful for removal and for this feature
Buying stacks instead of spam clicking
https://github.com/openfrontio/OpenFrontIO/pull/4640

Start by tracing how structures are rendered and how right-click targeting is handled, then review the related buying-stacks work in PR 4640. Done means hovering over your own structure produces a subtle icon-contrast change and makes removal or stack buying reliably target that exact structure.

Written by the indexing model from the issue text.