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/Makefile

@@ -6,7 +6,7 @@ WASM_OUT    := frontend/static/core.wasm
 WASM_EXEC   := frontend/static/wasm_exec.js
 TINYGO_ROOT := $(shell tinygo env TINYGOROOT 2>/dev/null)
 FBS_OUT     := frontend/src/proto/galaxy/fbs
-FBS_INPUTS  := ../pkg/schema/fbs/common.fbs ../pkg/schema/fbs/lobby.fbs ../pkg/schema/fbs/user.fbs ../pkg/schema/fbs/report.fbs ../pkg/schema/fbs/order.fbs
+FBS_INPUTS  := ../pkg/schema/fbs/common.fbs ../pkg/schema/fbs/lobby.fbs ../pkg/schema/fbs/user.fbs ../pkg/schema/fbs/report.fbs ../pkg/schema/fbs/order.fbs ../pkg/schema/fbs/diplomail.fbs
 
 help:
 	@echo "ui targets:"