feat(inbox): auto-scroll to comment from notification and add jump-to-bottom button

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.

server/internal/handler/reaction.go

@@ -92,6 +92,7 @@ func (h *Handler) AddReaction(w http.ResponseWriter, r *http.Request) {
 		"issue_id":            issueID,
 		"issue_title":         issueTitle,
 		"issue_status":        issueStatus,
+		"comment_id":          uuidToString(comment.ID),
 		"comment_author_type": comment.AuthorType,
 		"comment_author_id":   uuidToString(comment.AuthorID),
 	})