Phase 28 (Step 2): FBS schemas + message-type constants for mail

Adds the wire schema for the eight `user.games.mail.*` ConnectRPC
commands together with the shared payload types (`MailMessage`,
`MailRecipientState`, `MailBroadcastReceipt`). Send-request tables
carry the optional `recipient_race_name` introduced in Step 1.

Drops:

- `pkg/schema/fbs/diplomail.fbs` — schema sources;
- `pkg/schema/fbs/diplomail/*.go` — generated Go bindings (flatc
  `--go --go-module-name galaxy/schema/fbs`);
- `pkg/model/diplomail/diplomail.go` — message-type catalog used by
  the gateway router;
- `ui/frontend/src/proto/galaxy/fbs/diplomail/*.ts` — generated TS
  bindings consumed by the upcoming UI client wrapper;
- `ui/Makefile` `FBS_INPUTS` extended to pick the new schema up on
  the next `make -C ui fbs-ts` run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/src/proto/galaxy/fbs/diplomail.ts

@@ -0,0 +1,23 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+/* eslint-disable @typescript-eslint/no-unused-vars, @typescript-eslint/no-explicit-any, @typescript-eslint/no-non-null-assertion */
+
+export { AdminRequest, AdminRequestT } from './diplomail/admin-request.js';
+export { AdminResponse, AdminResponseT } from './diplomail/admin-response.js';
+export { BroadcastRequest, BroadcastRequestT } from './diplomail/broadcast-request.js';
+export { BroadcastResponse, BroadcastResponseT } from './diplomail/broadcast-response.js';
+export { DeleteRequest, DeleteRequestT } from './diplomail/delete-request.js';
+export { DeleteResponse, DeleteResponseT } from './diplomail/delete-response.js';
+export { InboxRequest, InboxRequestT } from './diplomail/inbox-request.js';
+export { InboxResponse, InboxResponseT } from './diplomail/inbox-response.js';
+export { MailBroadcastReceipt, MailBroadcastReceiptT } from './diplomail/mail-broadcast-receipt.js';
+export { MailMessage, MailMessageT } from './diplomail/mail-message.js';
+export { MailRecipientState, MailRecipientStateT } from './diplomail/mail-recipient-state.js';
+export { MessageGetRequest, MessageGetRequestT } from './diplomail/message-get-request.js';
+export { MessageGetResponse, MessageGetResponseT } from './diplomail/message-get-response.js';
+export { ReadRequest, ReadRequestT } from './diplomail/read-request.js';
+export { ReadResponse, ReadResponseT } from './diplomail/read-response.js';
+export { SendRequest, SendRequestT } from './diplomail/send-request.js';
+export { SendResponse, SendResponseT } from './diplomail/send-response.js';
+export { SentRequest, SentRequestT } from './diplomail/sent-request.js';
+export { SentResponse, SentResponseT } from './diplomail/sent-response.js';