feat: 一键安装支持自选游戏目录

StarLight 实例一键安装此前固定落到启动器默认目录，无法选择游戏数据位置。
现复用创建流程的外部游戏目录能力：点击一键安装后弹出目录选择框，
只允许修改游戏目录，其余参数仍由服务器整合包决定。

- hosted::create 接受 game_dir_root，拼为 <root>/<包名> 作为 game_dir_override
  （刻意避开 versions/ 布局，该结构被外部直链实例检测占用）
- 新增顶层命令 get_launcher_root_dir，作为弹窗默认值（可执行文件所在目录）
- 新增 HostedGameDirModal 弹窗：只读路径框 + 浏览 + 默认位置 + 安装预览
- 补齐 en-US / zh-CN / zh-TW 三语文案

hosted_create 增加 game_dir_root 参数，前端 hostedCreate/install 同步透传。

apps/app-frontend/src/locales/en-US/index.json

@@ -29,6 +29,14 @@
   "app.hosted-packs.auto-installing": { "message": "Downloading and installing the server modpack…" },
   "app.hosted-packs.auto-description": { "message": "Download and automatically install the modpack from StarLight to play on the StarLight server with one click." },
   "app.hosted-packs.auto-install": { "message": "Install the StarLight modpack" },
+  "app.hosted-install.game-dir.header": { "message": "Choose game directory" },
+  "app.hosted-install.game-dir.description": { "message": "StarLight instance data (mods, saves, configs, resource packs) is stored in an external game directory. Pick a root folder — the modpack gets its own subfolder inside it." },
+  "app.hosted-install.game-dir.label": { "message": "Game directory root" },
+  "app.hosted-install.game-dir.browse": { "message": "Browse" },
+  "app.hosted-install.game-dir.no-selection": { "message": "No folder selected" },
+  "app.hosted-install.game-dir.reset-default": { "message": "Use default location" },
+  "app.hosted-install.game-dir.preview": { "message": "Game files will be installed to: {path}" },
+  "app.hosted-install.game-dir.confirm": { "message": "Install" },
   "app.onboarding.instance-mode.title": { "message": "Choose your instance type" },
   "app.onboarding.instance-mode.description": { "message": "StarLight automatically installs the administrator-selected modpack and versions, then checks and completes updates before every launch. A StarLight login is required. Choose Local to select your own versions and modpacks for other servers or single-player." },
   "app.instance-mode.title": { "message": "Instance type" },