[Gameplay/UI] Add in-game Light Level Overlay (like MiniHUD) with F3 + L

It would be very helpful to have an in-game light level overlay that shows where mobs can spawn — similar to what mods like MiniHUD or Light Overlay offer.

I suggest adding a simple toggle (e.g., F3 + L) to show light levels directly on the blocks:

• Red: mobs can spawn (light level 0–7)

• Yellow: mobs can spawn at night

• Green: safe (light level 8+)

This would be extremely useful in survival mode, base-building, and resource management, without needing external mods or cheats.

It improves accessibility and helps newer players understand how mob spawning works.