release: v4.9.0 — full redesign + reworked offline mode

Ground-up visual redesign across the entire surface (landing, connection
setup, chat header, security verification report, file transfer, PWA
install/update/offline dialogs).

Offline reworked: store-and-forward queue (send while offline → queued,
delivered on reconnect), WhatsApp-style per-message delivery status
(sending/sent/delivered/not-sent) via delivery receipts, offline buffering
for messages to an offline peer, and offline state no longer leaking into
the connection indicator. Resilient chunked file transfer with retransmission
and auto-save. README + screenshots added.

SECURITY_DISCLAIMER.md

@@ -22,6 +22,6 @@ SecureBit.chat is intended for legitimate private communication, journalism, res
 
 ## Current release
 
-- Product release: `v4.8.20`
+- Product release: `v4.8.21`
 - Protocol version: `4.1`
 - Last updated: May 17, 2026