fix(battle-viewer): unblock synthetic-game battle load

The Phase 28 ConnectRPC migration of the battle viewer added a
guard in `lib/active-view/battle.svelte` that waits for the
surrounding layout to publish a `GalaxyClient` before issuing the
fetch. The in-game shell layout deliberately skips
`galaxyClient.set(...)` on the synthetic branch (gateway is not
reachable in synthetic mode), so for any battle opened from a
synthetic-report game the viewer sat on "loading battle…"
forever — `fetchBattle` was never called, so the synthetic-fixture
short-circuit it carries was unreachable.

Let the guard skip synthetic ids: `fetchBattle` already resolves
those through `lookupSyntheticBattle` and never touches the
client, so its signature widens to `GalaxyClient | null` and the
synthetic path passes `null`. The live path still waits for the
handle as before; a `null` client on the live path now fails
fast with a transport-level `BattleFetchError` instead of silently
sitting on `loading`.

Tests:
- Existing "loading placeholder" smoke now uses a non-synthetic
  game id so it keeps asserting the live-path wait.
- Two new cases pin the synthetic behaviour: missing fixture →
  `battle-not-found`; registered fixture → `BattleViewer` mounts.

Docs:
- `docs/FUNCTIONAL.md` §6.5 still described the pre-Phase-28
  raw REST path. Updated to the signed ConnectRPC command and
  noted the synthetic short-circuit. Russian mirror updated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/src/api/battle-fetch.ts

@@ -73,13 +73,16 @@ const RESULT_CODE_OK = "ok";
 /**
  * fetchBattle returns the `BattleReport` for the supplied game, turn,
  * and battle id. In synthetic-report mode (DEV / e2e) the lookup is
- * served from `synthetic-battle.ts`; otherwise the function calls the
- * `user.games.battle` ConnectRPC command through the supplied
- * `GalaxyClient`. Throws `BattleFetchError` with the upstream HTTP
- * status (or `0` for transport-level failures) on error.
+ * served from `synthetic-battle.ts` and the `client` argument is
+ * ignored; the in-game shell's synthetic branch deliberately never
+ * publishes a `GalaxyClient`, so callers pass `null` on that path.
+ * Otherwise the function calls the `user.games.battle` ConnectRPC
+ * command through the supplied `GalaxyClient` and throws
+ * `BattleFetchError` with the upstream HTTP status (or `0` for
+ * transport-level failures) on error.
  */
 export async function fetchBattle(
-	client: GalaxyClient,
+	client: GalaxyClient | null,
 	gameId: string,
 	turn: number,
 	battleId: string,
@@ -92,6 +95,10 @@ export async function fetchBattle(
 		return fixture;
 	}
 
+	if (client === null) {
+		throw new BattleFetchError(0, "GalaxyClient is unavailable");
+	}
+
 	const payload = encodeRequest(gameId, turn, battleId);
 	const result = await client.executeCommand(MESSAGE_TYPE, payload);
 	if (result.resultCode !== RESULT_CODE_OK) {