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>

pkg/model/report/messages.go

@@ -9,6 +9,13 @@ import "github.com/google/uuid"
 // `Report`.
 const MessageTypeUserGamesReport = "user.games.report"
 
+// MessageTypeUserGamesBattle is the authenticated gateway message type
+// used to fetch one battle report through
+// `GET /api/v1/user/games/{game_id}/battles/{turn}/{battle_id}`. The
+// signed payload is a FlatBuffers `GameBattleRequest`; the response is
+// a FlatBuffers `BattleReport`.
+const MessageTypeUserGamesBattle = "user.games.battle"
+
 // GameReportRequest is the typed payload of MessageTypeUserGamesReport.
 // `GameID` selects the target game (the message_type alone is not
 // enough; this scope is per-game) and `Turn` selects the requested
@@ -20,3 +27,19 @@ type GameReportRequest struct {
 	// Turn is the zero-based turn number whose report is requested.
 	Turn uint `json:"turn"`
 }
+
+// GameBattleRequest is the typed payload of MessageTypeUserGamesBattle.
+// `GameID` selects the target game; `Turn` is the turn the battle
+// happened at (the engine partitions battles by turn for cheap lookup);
+// `BattleID` is the in-game identifier returned in the report's
+// battle-summary list. All three fields are required.
+type GameBattleRequest struct {
+	// GameID identifies the game the battle belongs to.
+	GameID uuid.UUID `json:"game_id"`
+
+	// Turn is the turn number the battle happened at.
+	Turn uint `json:"turn"`
+
+	// BattleID is the engine-assigned id of the battle to fetch.
+	BattleID uuid.UUID `json:"battle_id"`
+}