#42830 feat: add `ITEM_MEDIA_SIZE` for `ItemMedia` avatars

Proposed changes (including videos or screenshots)

Adds ITEM_MEDIA_SIZE to @rocket.chat/ui-avatar: the avatar size an ItemMedia holds at each list density (condensed, medium, extended), matching the sidebar view modes. Lists migrating to Item size their avatars with it instead of hardcoding x20/x28/x36.

Issue(s)

Part of the Item migration. Used by #42818, #42819, #42820 and #42832, which carry this commit at the bottom until it lands.

Further comments

No consumer yet in this PR; no changeset, since @rocket.chat/ui-avatar is private.

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Summary by CodeRabbit

• New Features
  • Added shared avatar size options for condensed, medium, and extended list densities.