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/frontend/src/lib/active-view/battle.svelte

@@ -25,6 +25,10 @@ viewer keeps its prop-driven contract.
 		RENDERED_REPORT_CONTEXT_KEY,
 		type RenderedReportSource,
 	} from "$lib/rendered-report.svelte";
+	import {
+		GALAXY_CLIENT_CONTEXT_KEY,
+		type GalaxyClientHandle,
+	} from "$lib/galaxy-client-context.svelte";
 	import {
 		MapShipClassLookup,
 		type ShipClassLookup,
@@ -46,6 +50,9 @@ viewer keeps its prop-driven contract.
 	const rendered = getContext<RenderedReportSource | undefined>(
 		RENDERED_REPORT_CONTEXT_KEY,
 	);
+	const galaxyClient = getContext<GalaxyClientHandle | undefined>(
+		GALAXY_CLIENT_CONTEXT_KEY,
+	);
 
 	const shipClassLookup = $derived.by<ShipClassLookup>(() => {
 		const map = new Map<string, ShipClassRef>();
@@ -85,8 +92,16 @@ viewer keeps its prop-driven contract.
 			state = { kind: "not_found" };
 			return;
 		}
+		const client = galaxyClient?.client ?? null;
+		if (!client) {
+			// Layout populates the client after the boot Promise.all
+			// resolves; stay in `loading` so the effect re-runs once
+			// the handle becomes non-null.
+			state = { kind: "loading" };
+			return;
+		}
 		state = { kind: "loading" };
-		fetchBattle(gameId, turn, battleId)
+		fetchBattle(client, gameId, turn, battleId)
 			.then((report) => {
 				state = { kind: "ready", report };
 			})