fix(activity): address code review feedback and improve timeline UX

- Extract shared timeAgo utility, remove duplicates from comment-card and issue-detail
- Remove unused replies prop from CommentCard
- Fix recursive delete to remove all descendant replies, not just direct children
- Improve formatActivity with human-readable status/priority labels and actor names
- Validate parent comment exists and belongs to same issue before creating reply
- Add priority_changed activity recording in activity listeners
- Fix activity SQL query to sort ASC (was DESC, then re-sorted in handler)
- Fix reply-input layout alignment and test submit button selector
- Minor: .gitignore additions, button dark mode aria-expanded fix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/internal/handler/comment.go

@@ -94,6 +94,11 @@ func (h *Handler) CreateComment(w http.ResponseWriter, r *http.Request) {
 	var parentID pgtype.UUID
 	if req.ParentID != nil {
 		parentID = parseUUID(*req.ParentID)
+		parent, err := h.Queries.GetComment(r.Context(), parentID)
+		if err != nil || uuidToString(parent.IssueID) != issueID {
+			writeError(w, http.StatusBadRequest, "invalid parent comment")
+			return
+		}
 	}
 
 	comment, err := h.Queries.CreateComment(r.Context(), db.CreateCommentParams{