feat(activity): fall back to name for custom activities

[shiftinv]

Summary

Bots can now set a custom status using the state field in type 4 activities.

This already works using activity=CustomActivity(state="Now with 200% more bees!", name="Custom Status"), but is unnecessarily complicated. name is required but not displayed, it just has to be non-empty.
To simplify things, we now just make state fall back to name if unset, such that CustomActivity(name="...") works as-is.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)