Nested Replies Now Supported ✅

I’ve added proper nested reply support to [Postr](https://apps.nostrwat.ch/postr/) client.

Replies now preserve thread ancestry using correct root + reply tags (NIP-10 compliant).

✔ Reply to root posts
✔ Reply to replies
✔ Automatic root detection
✔ No architecture rewrite

Threading now behaves like major clients.