v3.28.6: keep the PoR delivered with a 91xx ENVELOPE answer

A card may deliver the response packet (the PoR) in the GET RESPONSE of
an ENVELOPE that answered 61xx and signal a proactive command pending in
the same answer (91xx - the CAT success state, not an error).
_send_envelope ran the chain and then cleared the response data
(`data, sw = '', '9000'`), so the operation reported no PoR although the
card had returned one (live 2026-10-14: a GP DELETE answered 6113, the
GET RESPONSE delivered the PoR with 9130, and the delete verdict showed
"ok (no PoR)").

The delivered bytes are kept now; the chain still runs and the exchange
reports 9000.  Test: the delivered PoR survives the chain (verified to
fail on the old code).  917 frontend + 928 Python; sw cache simple-v444.

frontend/index.html

@@ -1910,7 +1910,7 @@
 // ===== Version =====
 // Single source of truth for the PWA version: shown in the header and used
 // by the server version check in pysimConnect().
-const SIMPLE_VERSION = '3.28.5';
+const SIMPLE_VERSION = '3.28.6';
 document.getElementById('app-version').textContent = 'v' + SIMPLE_VERSION;
 
 // ===== view history (the browser Back/Forward buttons) =====