feat: add drag-and-drop reordering for message queues + fix ghost unread indicators

- Add drag-and-drop reordering to Compose and Groups message sequence queues
- Fix critical dragstart bug: e.target in delegated handler is the draggable div
  (not the clicked child), so use mousedown flag to detect handle-origin drags
- Fix dragover guard: skip if no active drag (prevents interference with text selection)
- Fix placeholder flicker on dragleave by checking e.relatedTarget container membership
- Fix redundant DOM insertions: skip placeholder re-insert when already in correct spot
- Add load progress status bar (Loading chats / Updating X group names)
- Remove Updating... bar from 5-second background poll
- Fix ghost unread indicators on groups: clear unreadCount optimistically before API call
- Fix stale-jid guards across chatPollMessages, chatFetchOlderFromApi, chatSendMediaMsg
- Fix double-send race with _sending lock and finally-block release
- Add visibilitychange listener for immediate refresh on tab return
- Fix chatMarkRead to use per-message remoteJid instead of conversation jid

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: erancybersec