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

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1781648539643",
-  "buildVersion": "1781648539643",
-  "appVersion": "4.8.11",
-  "buildTime": "2026-06-16T22:22:19.692Z",
-  "buildId": "1781648539643-9244250",
-  "gitHash": "9244250",
+  "version": "1781732923420",
+  "buildVersion": "1781732923420",
+  "appVersion": "4.8.12",
+  "buildTime": "2026-06-17T21:48:43.476Z",
+  "buildId": "1781732923420-be1d02f",
+  "gitHash": "be1d02f",
   "generated": true,
-  "generatedAt": "2026-06-16T22:22:19.693Z"
+  "generatedAt": "2026-06-17T21:48:43.478Z"
 }