release: v4.8.11 file transfer reliability fix

fix(file-transfer): size chunks under the 64KB SCTP message limit

Each 64KB chunk became a ~87KB AES-GCM+Base64 file_chunk message,
exceeding WebRTC's 64KB SCTP message-size floor. The consent handshake
(small messages) succeeded, but no chunk was ever delivered on Safari
and cross-browser connections whose SDP omits a=max-message-size, so
files never transferred. Send chunk size is now 16KB (~22KB on the
wire); inbound chunks up to 64KB stay accepted for backward compat.

fix(file-transfer): make MIME advisory, drive validation by extension

The client-supplied MIME type is easily spoofed and varies across
browsers/OSes, yet was a hard gate: files with an empty MIME or a
cross-OS variant (application/x-zip-compressed, image/jpg) were wrongly
rejected. Extension allow-list plus BLOCKED_EXTENSIONS is now the
boundary; a blatantly foreign MIME on a safe extension is still rejected
and per-type size limits still apply.

manifest.json

@@ -1,5 +1,5 @@
 {
-  "name": "SecureBit.chat v4.8.10 - ECDH + DTLS + SAS",
+  "name": "SecureBit.chat v4.8.11 - ECDH + DTLS + SAS",
   "short_name": "SecureBit",
   "description": "P2P messenger with ECDH + DTLS + SAS security, military-grade cryptography and Lightning Network payments",
   "start_url": "./",