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

src/app.jsx

@@ -276,7 +276,7 @@ import { loadIceSettings, saveIceSettings, clearIceSettings } from './network/ic
                             }),
                             React.createElement('div', {
                                 key: 'text',
-                                className: "flex-1"
+                                className: "flex-1 min-w-0"
                             }, [
                                 React.createElement('div', {
                                     key: 'message',
@@ -2071,7 +2071,7 @@ import { loadIceSettings, saveIceSettings, clearIceSettings } from './network/ic
                             }
                         }
         
-                        handleMessage(' SecureBit.chat Enhanced Security Edition v4.8.10 - ECDH + DTLS + SAS initialized. Ready to establish a secure connection with ECDH key exchange, DTLS fingerprint verification, and SAS authentication to prevent MITM attacks.', 'system');
+                        handleMessage(' SecureBit.chat Enhanced Security Edition v4.8.12 - ECDH + DTLS + SAS initialized. Ready to establish a secure connection with ECDH key exchange, DTLS fingerprint verification, and SAS authentication to prevent MITM attacks.', 'system');
         
                         const handleBeforeUnload = (event) => {
                             if (event.type === 'beforeunload' && !isTabSwitching) {