Show preview of user's profile

**Current Behavior**
A user has to click on another user's name to see their profile.

**Expected Behavior**
A user can hover over another user's name to see a preview of their profile in a popup.
This should be done in two places:
1. When a user is mentioned in a post (#122)/comment.
2. When a user is part of a transaction post, e.g. X sent token A to Y

Start by locating the UI components that render user mentions in posts and comments, then find where transaction posts display their participants. Trace how user names are linked in both places and determine the existing popup or profile-preview patterns. Done means hovering over a user in each context shows a profile preview without requiring navigation.

Written by the indexing model from the issue text.