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/pkg/db/queries/activity.sql

@@ -1,7 +1,7 @@
 -- name: ListActivities :many
 SELECT * FROM activity_log
 WHERE issue_id = $1
-ORDER BY created_at DESC
+ORDER BY created_at ASC
 LIMIT $2 OFFSET $3;
 
 -- name: CreateActivity :one