Introduces an exclusive file lock (FileShare.None on wallet.json.lock)
held for the duration of a wallet session. A second instance trying to
open the same file receives null from TryAcquire and sees a localized
error message; UnauthorizedAccessException (OS permission issues)
propagates separately so the UI can show a distinct message.
The .lock file is deleted on Dispose (best-effort); the real guard is
the open FileStream, which the OS releases automatically on crash.
davide
0f8a764a44