Rewrite UI

[umut-sahin]

Built-in Bevy UI have some issues at the moment:

• It's VERY verbose, we have about 5k lines of code in the main UI module, and it's not even close to ready yet.
• It's missing some important functionality:
  • No world-space UI (e.g., over the head health bars, damage numbers in a specific location in the world, etc.)
  • No widget hierarchy (i.e., to navigate with the keyboard)

There are two replacement options:

• https://github.com/bytestring-net/bevy_lunex
• https://github.com/UmbraLuminosa/sickle_ui

We should inspect both, make a decision, and rewrite the use using the selected library.