[Due for payment 2025-11-17] [Due for payment 2025-11-14] [Avatar Customization] Post Release Tidy Up

[grgia]

Main Issue

This issue is to track any kinds of updates that might fall under "Leave the code better than ya found it".

1. Remove beta from FE & Web

2. Clean up UserUtils to use CustomAvatarCatalog.

[jmusial]

Ready in PR:

• Remove beta

Ready in a draft PR

• Add docs for person avatar naming conventions
• Refactor UserUtils avatar related functions to use CustomAvatarCatalog. (Basically make it the source of truth for User avatars)
• Align Avatar preview behaviour for default, custom and letter avatars (TBC what should it be)
• Const, general cleanup refactor

TODO:

• Make letter avatars display as svgs locally and align preview behaviour

[grgia]

Perf thank you @jmusial !

[jmusial]

Added PR for beta removal

[jmusial]

Added a draft PR with utils refactor. On second though it's gonna be better to do #73173 in separate PR as this one is big enough already.

Draft bc it depends on #73412