bug: duplicate reaction avatars and keys when toggling reactions

[AbhiVarde]

When a user adds a reaction, the avatar and count sometimes appear duplicated. This also triggers a React console error:

Encountered two children with the same key <username>. Keys should be unique so that components maintain their identity across updates.

Root cause in apps/web/src/components/shared/reaction-display.tsx:

1. getUsersForReaction does not deduplicate users, so a user can appear twice in the list if they appear multiple times in reactionUsers state
2. The optimistic update in handleToggleReaction adds the current user to reactionUsers without checking if they already exist

Steps to reproduce:

1. Open any PR
2. Go to Conversation tab
3. Add a reaction to any comment
4. Observe duplicate avatar and count, and React key warning in console