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>

pkg/schema/fbs/diplomail/MessageGetResponse.go

@@ -0,0 +1,65 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package diplomail
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type MessageGetResponse struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsMessageGetResponse(buf []byte, offset flatbuffers.UOffsetT) *MessageGetResponse {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &MessageGetResponse{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishMessageGetResponseBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsMessageGetResponse(buf []byte, offset flatbuffers.UOffsetT) *MessageGetResponse {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &MessageGetResponse{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedMessageGetResponseBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *MessageGetResponse) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *MessageGetResponse) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *MessageGetResponse) Message(obj *MailMessage) *MailMessage {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		x := rcv._tab.Indirect(o + rcv._tab.Pos)
+		if obj == nil {
+			obj = new(MailMessage)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
+func MessageGetResponseStart(builder *flatbuffers.Builder) {
+	builder.StartObject(1)
+}
+func MessageGetResponseAddMessage(builder *flatbuffers.Builder, message flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(message), 0)
+}
+func MessageGetResponseEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}