refactor(web): expose avatar primitives for composition

[lyzno1]

Summary

• expose AvatarRoot, AvatarImage, and AvatarFallback from the base avatar module
• export shared avatar size class helpers so composed usages can keep the same size tokens
• keep the default Avatar API intact while rebuilding it on top of the exported primitives

Testing

• ./node_modules/.bin/eslint app/components/base/avatar/index.tsx

Notes

• the repository pre-commit type-check:tsgo currently fails on an unrelated existing error in web/app/components/base/features/new-feature-panel/annotation-reply/score-slider/base-slider/index.tsx

[Copilot]

Pull request overview

This PR refactors the base web Avatar component to expose composable avatar primitives and shared size-class helpers, while keeping the existing Avatar component API intact.

Changes:

• Export AvatarRoot, AvatarImage, and AvatarFallback from the base avatar module for composition.
• Export shared size-token helpers (avatarSizeClasses, AvatarSize, getAvatarSizeClassNames) so composed avatars can reuse the same sizing.
• Rebuild the default Avatar component implementation on top of the exported primitives and shared class constants.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.98%. Comparing base (5f82ccc) to head (803079f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #34057   +/-   ##
=======================================
  Coverage   79.97%   79.98%           
=======================================
  Files        4609     4609           
  Lines      182612   182618    +6     
  Branches    35573    35573           
=======================================
+ Hits       146049   146064   +15     
+ Misses      33345    33336    -9     
  Partials     3218     3218           

Flag	Coverage Δ
web	81.33% <100.00%> (+0.01%)	⬆️

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