feat(UI): Change floodlight symbol to 'T'

[Soadreqm]

Checklist

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• I wrote the PR title in conventional commit format.
• I ran the code formatter.
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Purpose of change

Grid floodlight symbol was difficult to see on a floor when using ASCII graphics.

Describe the solution

Changed symbol of f_grid_floodlight (all variants) from . to T, same as the floor lamp. Also changed the "on" state color from white to yellow, to provide a visual indicator that the floodlight is active.

Describe alternatives you've considered

The t_utility_light that appears in labs has the same issue, being basically invisible on a concrete floor, but I think it's less of a problem there. They don't use grid power, so there's rarely any reason to turn them off.

Testing

Looked at a built floodlight, turned it on and off.

Additional context

[chaosvolt]

I presumably copypasted it from utility light terrain, where it being a . sorta made sense because it had zero impact as an obstacle and was basically just a floor but bright. Thanks for spotting that!