feat: shared volume settings with a per-game override

- Music, sound and voice volumes are shared by all games by default; a game that
  has never been launched with its own volumes plays with the shared ones
- The sound section of the player settings has a "own volume for this game" switch:
  turning it on starts from the current values and then the sliders change only this
  game, turning it off removes the override and returns to the shared volumes
- The override belongs to the game (game_id from /api/versions/{id}), so forks and mods
  share it; it falls back to the version id when the id is unavailable
- Reading speed and auto delay stay shared
- volume-store.ts keeps the storage logic (damaged data ignored, values clamped,
  unavailable storage tolerated) and has its own tests

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/player/index.ts

@@ -43,9 +43,12 @@ export async function launch(root: HTMLElement, versionId: number, onExit: () =>
     if (!load) throw new Error(`Неизвестная среда исполнения игры: ${game.runtime}`);
     return (await load())({ root, game, manifest, modules, url, tr, onExit, versionId, saveApi: (id) => saveApi(id, cloud) });
   }
+  // volume belongs to the game, not to one of its versions (forks and mods share it)
+  const gameId = await getJSON<{ game_id: number }>(`/api/versions/${versionId}`).then((v) => v.game_id, () => null);
   const engine = new Engine(root, game, manifest, modules, saveApi(versionId, cloud),
                             `pgvn:persistent:${versionId}`, onExit, tr,
-                            (code) => loadTranslator(game, manifest, code));
+                            (code) => loadTranslator(game, manifest, code),
+                            gameId === null ? `pgvn:audio:version:${versionId}` : `pgvn:audio:game:${gameId}`);
   const at = url.get("at");
   if (at) engine.startAt(at);
   else engine.start();