When clicking an inbox notification, the issue detail now scrolls to and
briefly highlights the relevant comment. Also adds a floating "Jump to
bottom" button on issue pages with long timelines.
Backend: store comment_id in inbox notification details for new_comment
and reaction_added events. Frontend: pass highlightCommentId through to
IssueDetail, add id attributes to comment elements, and track scroll
position for the jump-to-bottom button.
575bbd7f60