release: v4.8.12 chat notification & file-transfer UI fixes

fix(file-transfer): announce received file once, not many times

The per-transfer lock used a single `if` check, so when 3+ chunk
operations queued on the same fileId they awaited the same in-flight
lock and then ran concurrently, breaking assembly atomicity. The lock
now loops until the slot is free (true serialization) and file assembly
is idempotent, so `File received` shows exactly once per file.

fix(verification): stop duplicate connection-setup system messages

handleVerificationBothConfirmed had no guard, so when both peers sent
verification_both_confirmed symmetrically one side ran both the local
detection path and the peer-notification path, emitting "Both parties
confirmed!" and the verified transition (and "Secure connection
established") twice. It now bails out if both confirmations are already
recorded.

fix(ui): wrap long DTLS fingerprint inside the chat bubble

The message text column is a flex child with default min-width:auto, so
the long unbroken fingerprint overflowed. Added min-w-0 so break-words
can wrap it.

chore(release): bump version to 4.8.12 in header, init banner, manifest

manifest.json

@@ -1,5 +1,5 @@
 {
-  "name": "SecureBit.chat v4.8.11 - ECDH + DTLS + SAS",
+  "name": "SecureBit.chat v4.8.12 - ECDH + DTLS + SAS",
   "short_name": "SecureBit",
   "description": "P2P messenger with ECDH + DTLS + SAS security, military-grade cryptography and Lightning Network payments",
   "start_url": "./",