feat(fedi): hide the leading @mention in displayed comments
A federated reply carries a leading @user@domain mention (the person replied to).
Strip it for display (Mastodon does the same) so a comment reads 'dope tekening ouwe'
not '@jason@… dope …'. Applied to inbound + outbound reply nodes, the
My-replies list, and the notifications preview. stripLeadingMentions keeps the <p>
wrapper and handles both <a>mention</a> links and plain-text mentions.
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