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/components/ui/Header.jsx

@@ -539,7 +539,7 @@ const EnhancedMinimalHeader = ({
                         React.createElement('p', {
                             key: 'subtitle',
                             className: 'text-xs sm:text-sm text-muted hidden sm:block'
-                        }, 'End-to-end freedom v4.8.10')
+                        }, 'End-to-end freedom v4.8.12')
                     ])
                 ]),