Strip LC_* before mosh ssh handshake

[binaricat]

Summary

• macOS Terminal/iTerm export LC_CTYPE=UTF-8 (a bare value, not a real locale name). System ssh_config has SendEnv LC_*, so when netcatty spawns the system ssh for mosh handshake, that value leaks to the remote and every login prints:

  -bash: warning: setlocale: LC_CTYPE: cannot change locale (UTF-8): No such file or directory
  

• Tried -o "SendEnv -LC_*" first but OpenSSH processes -o before the system config, so the - removal happens against an empty list and the system SendEnv LC_* re-adds it.
• Fix: scrub all LC_* keys from the env passed to the spawned ssh child in `startMoshSessionViaHandshake`. mosh-server already sets the locale it needs via the `LC_ALL=...` prefix.

Test plan

• mosh-connect to a Debian/Ubuntu host that does not have `UTF-8` (or `en_US.UTF-8`) locale generated; confirm bash no longer prints the `setlocale` warning at login.
• Confirm UTF-8 characters render correctly in the mosh session (Chinese filenames, emoji, etc).
• `node --test electron/bridges/moshHandshake.test.cjs` still passes.

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