battle-fetch: migrate to user.games.battle ConnectRPC command

The Phase 27 BattleViewer was the last UI surface still issuing raw
fetch() against the backend REST contract (`/api/v1/user/games/...
/battles/...`). The dev-deploy gateway never proxied that path, so
the viewer worked only in tools/local-dev/. Move it onto the signed
ConnectRPC channel every other authenticated surface already uses.

Wire pieces:
- FBS GameBattleRequest in pkg/schema/fbs/battle.fbs, regenerated
  Go + TS bindings.
- MessageTypeUserGamesBattle constant + GameBattleRequest struct in
  pkg/model/report/messages.go.
- pkg/transcoder/battle.go gains GameBattleRequestToPayload and
  PayloadToGameBattleRequest helpers.
- gateway games_commands.go switches on the new message type and
  GETs /api/v1/user/games/{id}/battles/{turn}/{battle_id}; the JSON
  response is re-encoded as a FlatBuffers BattleReport before being
  returned. 404 from backend surfaces as the canonical `not_found`
  gateway error.
- ui/frontend/src/api/battle-fetch.ts now builds the FBS request,
  calls GalaxyClient.executeCommand, and decodes the FBS response
  into the existing UI shape (Record<string,string> race/ship maps,
  string-form UUID). BattleFetchError carries an HTTP-style status
  derived from the result code so the active-view's not_found branch
  keeps working.
- battle.svelte pulls the GalaxyClient from the in-game shell
  context. While the layout's boot Promise.all is in flight the
  effect stays in `loading` until the client handle becomes
  non-null.
- ui/Makefile FBS_INPUTS gains battle.fbs.

Co-Authored-By: Claude Opus 4.7 <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 ../pkg/schema/fbs/diplomail.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 ../pkg/schema/fbs/battle.fbs
 
 help:
 	@echo "ui targets:"