feat(threads): support nested replies in thread API and panel

docs/api-reference.md

@@ -832,7 +832,7 @@ Response: `200` + `MessageRead[]`
 ### GET `/api/v1/messages/{message_id}/thread?limit=100`
 
 Auth required.  
-Returns root message + direct replies (basic thread view).  
+Returns root message + nested replies (thread subtree, BFS by reply links).  
 Response: `200` + `MessageRead[]`
 
 ### PUT `/api/v1/messages/{message_id}`

docs/core-checklist-status.md

@@ -16,7 +16,7 @@ Legend:
 7. Chat Creation - `DONE` (private/group/channel)
 8. Messages (base) - `DONE` (send/read/edit/delete/delete for all)
 9. Message Types - `PARTIAL` (text/photo/video/docs/audio/voice/circle; GIF/stickers via dedicated system missing)
-10. Reply/Quote/Threads - `PARTIAL` (reply + quote-like UI + basic thread panel, no deep nested thread model)
+10. Reply/Quote/Threads - `PARTIAL` (reply + quote-like UI + thread panel with nested replies, no dedicated full thread navigation yet)
 11. Forwarding - `PARTIAL` (single + bulk; "without author" missing)
 12. Pinning - `DONE` (message/chat pin-unpin)
 13. Reactions - `DONE`