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/message-get-response.ts

@@ -0,0 +1,77 @@
+// 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 */
+
+import * as flatbuffers from 'flatbuffers';
+
+import { MailMessage, MailMessageT } from '../diplomail/mail-message.js';
+
+
+export class MessageGetResponse implements flatbuffers.IUnpackableObject<MessageGetResponseT> {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):MessageGetResponse {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsMessageGetResponse(bb:flatbuffers.ByteBuffer, obj?:MessageGetResponse):MessageGetResponse {
+  return (obj || new MessageGetResponse()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsMessageGetResponse(bb:flatbuffers.ByteBuffer, obj?:MessageGetResponse):MessageGetResponse {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new MessageGetResponse()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+message(obj?:MailMessage):MailMessage|null {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? (obj || new MailMessage()).__init(this.bb!.__indirect(this.bb_pos + offset), this.bb!) : null;
+}
+
+static startMessageGetResponse(builder:flatbuffers.Builder) {
+  builder.startObject(1);
+}
+
+static addMessage(builder:flatbuffers.Builder, messageOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(0, messageOffset, 0);
+}
+
+static endMessageGetResponse(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createMessageGetResponse(builder:flatbuffers.Builder, messageOffset:flatbuffers.Offset):flatbuffers.Offset {
+  MessageGetResponse.startMessageGetResponse(builder);
+  MessageGetResponse.addMessage(builder, messageOffset);
+  return MessageGetResponse.endMessageGetResponse(builder);
+}
+
+unpack(): MessageGetResponseT {
+  return new MessageGetResponseT(
+    (this.message() !== null ? this.message()!.unpack() : null)
+  );
+}
+
+
+unpackTo(_o: MessageGetResponseT): void {
+  _o.message = (this.message() !== null ? this.message()!.unpack() : null);
+}
+}
+
+export class MessageGetResponseT implements flatbuffers.IGeneratedObject {
+constructor(
+  public message: MailMessageT|null = null
+){}
+
+
+pack(builder:flatbuffers.Builder): flatbuffers.Offset {
+  const message = (this.message !== null ? this.message!.pack(builder) : 0);
+
+  return MessageGetResponse.createMessageGetResponse(builder,
+    message
+  );
+}
+}