feat: display gravatars as user-images

[maxy-shpfy]

Gravatar support for user and message avatars

Contributes to https://github.com/Shopify/oasis-frontend/issues/682

UserAvatar now resolves a Gravatar image for a given email address rather than rendering an initials badge directly. It fetches the Gravatar URL (using d=404 so missing avatars return a 404 rather than a placeholder), caches the result indefinitely via React Query, and renders a fallback node when no avatar exists or the lookup is still in flight.

The initials badge previously baked into UserAvatar has been extracted into a standalone UserInitialsBadge component and passed as the fallback prop at the AppTopNav call site.

MessageAvatar in the chat view now accepts an optional email prop for human authors (sourced from message.author.id). When present, it delegates to UserAvatar with the role icon badge as the fallback, so human messages show a Gravatar when one exists and silently fall back to the existing icon badge when one does not.

Gravatar URL construction uses the Web Crypto API (crypto.subtle) for SHA-256 hashing, avoiding an md5 dependency or Node crypto usage in the browser.

[maxy-shpfy]

• feat: sync via ws participants in sessions #73
• feat: display gravatars as user-images #72 👈 (View in Graphite)
• main

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