feat(activity): unified activity timeline with comment reply support

Replace the comment-only list with a Linear-style unified timeline that
interleaves field changes and comments chronologically.

Backend:
- activity_listeners.go: records field changes (status, assignee, description,
  task completed/failed) to activity_log table on domain events
- Timeline API: GET /api/issues/{id}/timeline merges activity_log + comments
  sorted by created_at
- Comment reply: parent_id column + handler support for threading

Frontend:
- Unified timeline replaces comment list: activity entries as compact muted
  lines, comments as Card components with reply threading
- Filter toggle (All / Comments / Activity)
- Reply UI: inline editor under comments with Cancel/Reply buttons
- Real-time sync for activity:created + comment events
- 10 new Go tests, all passing

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

server/pkg/db/queries/comment.sql

@@ -8,8 +8,8 @@ SELECT * FROM comment
 WHERE id = $1;
 
 -- name: CreateComment :one
-INSERT INTO comment (issue_id, author_type, author_id, content, type)
-VALUES ($1, $2, $3, $4, $5)
+INSERT INTO comment (issue_id, author_type, author_id, content, type, parent_id)
+VALUES ($1, $2, $3, $4, $5, sqlc.narg(parent_id))
 RETURNING *;
 
 -- name: UpdateComment :one