[QoL] Auto filtering on challenge mode

[KimJeongSun]

What are the changes?

You no longer need to spend endless time searching for valid Pokémon in the starter select UI during challenge mode.

Why am I doing these changes?

I think this approach is way better. Does anyone disagree?

What did change?

In challenge mode, valid Pokémon will be pre-filtered according to the game mode.
( + The variable name starterContainer could be confusing, so I've corrected it to starterContainers. )

Screenshots/Videos

Screen.Recording.2024-08-03.at.2.54.49.PM.mp4

How to test the changes?

Checklist

• I'm using beta as my base branch
• There is no overlap with another PR?
• The PR is self-contained and cannot be split into smaller PRs?
• Have I provided a clear explanation of the changes?
• Have I considered writing automated tests for the issue?
• Have I tested the changes (manually)?
  • Are all unit tests still passing? (npm run test)
• Are the changes visual?
  • Have I provided screenshots/videos of the changes?

[DayKev]

Looks like it takes into account future/potential eligibility too, nice.

[CodeTappert]

Codewise it looks fine. I will try this out locally