Adds typed wrappers around `GalaxyClient.executeCommand` for the
eight Phase 28 mail RPCs. Each wrapper builds the matching
FlatBuffers request, decodes the response, and surfaces backend
errors through a dedicated `MailError` (mirroring `LobbyError`).
The compose helpers accept the recipient race name directly so the
UI can feed it straight from `report.races[].name` without a
membership lookup.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>