FE-1305: add avatar group to ds

[alex-e-leon]

🌟 What is the purpose of this PR?

Adds AvatarGroup component

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

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PR Summary

Medium Risk
Touches the published Avatar component (border styling and new context dependency) in addition to a large new interactive UI primitive, so standalone Avatar appearance may shift slightly at some sizes.

Overview
Adds AvatarGroup to @hashintel/ds-components for overlapping avatar stacks with optional max / total surplus badges, AvatarGroup.More for custom overflow content, spacing, lastOnTop, and group-level size / tone.

Avatar now reads AvatarGroupContext so children inherit group size/tone when omitted, and applies a shared avatarSize recipe so --avatar-size is set on both the avatar and the group root (needed for overlap/lift styling on parent wrappers). Avatar border width is unified to a size-based formula (/32 with clamp) instead of per-size overrides.

Hover/focus behavior uses column-based pointer hit targets, lift/cross-fade animations, and a temporary opaque clone layer to avoid visual flash. The component is exported from main.ts with a patch changeset.

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