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/components/instance/HostedGameDirModal.vue

@@ -0,0 +1,161 @@
+<template>
+	<NewModal
+		ref="modal"
+		:header="formatMessage(messages.header)"
+		max-width="560px"
+		:on-hide="handleHide"
+	>
+		<div class="flex flex-col gap-4">
+			<p class="m-0 text-secondary">
+				{{ formatMessage(messages.description) }}
+			</p>
+
+			<div class="flex flex-col gap-2">
+				<span class="font-semibold text-contrast">
+					{{ formatMessage(messages.gameDirLabel) }}
+				</span>
+				<div class="flex gap-2">
+					<StyledInput
+						class="flex-1"
+						:model-value="selectedPath"
+						readonly
+						:placeholder="defaultPath || formatMessage(messages.noSelection)"
+					/>
+					<ButtonStyled>
+						<button type="button" @click="browse">
+							<FolderOpenIcon />
+							{{ formatMessage(messages.browse) }}
+						</button>
+					</ButtonStyled>
+				</div>
+				<button
+					v-if="defaultPath && selectedPath !== defaultPath"
+					type="button"
+					class="self-start text-sm text-brand underline decoration-transparent underline-offset-2 transition-colors hover:decoration-current"
+					@click="selectedPath = defaultPath"
+				>
+					{{ formatMessage(messages.resetDefault) }}
+				</button>
+			</div>
+
+			<p v-if="previewPath" class="m-0 text-sm text-secondary break-all">
+				{{ formatMessage(messages.preview, { path: previewPath }) }}
+			</p>
+		</div>
+
+		<template #actions>
+			<div class="flex justify-end gap-2">
+				<ButtonStyled type="outlined">
+					<button type="button" @click="handleCancel">
+						<XIcon />
+						{{ formatMessage(commonMessages.cancelButton) }}
+					</button>
+				</ButtonStyled>
+				<ButtonStyled color="brand">
+					<button type="button" :disabled="!selectedPath" @click="handleConfirm">
+						<CheckIcon />
+						{{ formatMessage(messages.confirm) }}
+					</button>
+				</ButtonStyled>
+			</div>
+		</template>
+	</NewModal>
+</template>
+
+<script setup lang="ts">
+import { CheckIcon, FolderOpenIcon, XIcon } from '@modrinth/assets'
+import {
+	ButtonStyled,
+	commonMessages,
+	defineMessages,
+	injectFilePicker,
+	NewModal,
+	StyledInput,
+	useVIntl,
+} from '@modrinth/ui'
+import { invoke } from '@tauri-apps/api/core'
+import { computed, ref } from 'vue'
+
+const emit = defineEmits<{
+	(e: 'confirm', gameDirRoot: string): void
+	(e: 'cancel'): void
+}>()
+
+const { formatMessage } = useVIntl()
+const filePicker = injectFilePicker()
+const modal = ref<InstanceType<typeof NewModal>>()
+const defaultPath = ref('')
+const selectedPath = ref('')
+const accepted = ref(false)
+
+const messages = defineMessages({
+	header: {
+		id: 'app.hosted-install.game-dir.header',
+		defaultMessage: 'Choose game directory',
+	},
+	description: {
+		id: 'app.hosted-install.game-dir.description',
+		defaultMessage:
+			'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.',
+	},
+	gameDirLabel: {
+		id: 'app.hosted-install.game-dir.label',
+		defaultMessage: 'Game directory root',
+	},
+	browse: {
+		id: 'app.hosted-install.game-dir.browse',
+		defaultMessage: 'Browse',
+	},
+	noSelection: {
+		id: 'app.hosted-install.game-dir.no-selection',
+		defaultMessage: 'No folder selected',
+	},
+	resetDefault: {
+		id: 'app.hosted-install.game-dir.reset-default',
+		defaultMessage: 'Use default location',
+	},
+	preview: {
+		id: 'app.hosted-install.game-dir.preview',
+		defaultMessage: 'Game files will be installed to: {path}',
+	},
+	confirm: {
+		id: 'app.hosted-install.game-dir.confirm',
+		defaultMessage: 'Install',
+	},
+})
+
+const previewPath = computed(() => {
+	const base = (selectedPath.value ?? '').replace(/[\\/]+$/, '')
+	return base ? `${base}/<pack name>` : ''
+})
+
+async function show() {
+	if (!defaultPath.value) {
+		defaultPath.value = await invoke<string>('get_launcher_root_dir').catch(() => '')
+	}
+	if (!selectedPath.value) selectedPath.value = defaultPath.value
+	accepted.value = false
+	modal.value?.show()
+}
+
+async function browse() {
+	const picked = await filePicker.pickFolder?.()
+	if (picked?.path) selectedPath.value = picked.path
+}
+
+function handleCancel() {
+	modal.value?.hide()
+}
+
+function handleConfirm() {
+	accepted.value = true
+	modal.value?.hide()
+	emit('confirm', selectedPath.value)
+}
+
+function handleHide() {
+	if (!accepted.value) emit('cancel')
+}
+
+defineExpose({ show })
+</script>