This PR fixes WebSocket transfers that stall when unread data remains buffered.
Miniserv now drains buffered browser data before waiting for more network traffic.
The receive library avoids extra blocking reads and handles temporary read errors without disconnecting.
Tested and verified over HTTPS with Terminal and live stats on Rocky 10 and Ubuntu 26.04.
I discovered these issues while working on and testing new Cloudmin's SSH-based guest File Manager integration, which I decided to drop for now.