feat: 跨实例同步移植第8轮（前端 helpers + 同步设置 UI）

新增同步 helpers（synced-options / synced-packs / game-options），在 instance helper 中补充同步类型与 API。新增实例设置「同步」标签页（SyncSettings）及 SyncSource / SyncedPacks / servers / command-history 四个弹窗，并把 CheckCircleButton、TeleportOverflowMenu 从 ui 包导出。同步相关的 en-US / zh-CN 文案已补齐（各 62 条）。game-settings-modal 留待下一轮。

apps/app-frontend/src/components/ui/settings/instances/SyncSourceModal.vue

@@ -0,0 +1,181 @@
+<script setup lang="ts">
+import { RefreshCwIcon, SearchIcon, SpinnerIcon, XIcon } from '@modrinth/assets'
+import {
+	Avatar,
+	Button,
+	CheckCircleButton,
+	commonMessages,
+	defineMessages,
+	NewModal,
+	StyledInput,
+	useVIntl,
+} from '@modrinth/ui'
+import { computed, ref, useTemplateRef } from 'vue'
+
+import { getInstanceIconUrl } from '@/helpers/instance'
+
+const props = defineProps<{
+	title: string
+	description: string
+	sources: {
+		id: string
+		name: string
+		icon_path?: string | null
+		eligible: boolean
+	}[]
+	loading?: boolean
+	pending?: boolean
+	error?: boolean
+}>()
+
+const emit = defineEmits<{
+	confirm: []
+	close: []
+	retry: []
+}>()
+
+const selectedInstanceId = defineModel<string>({ required: true })
+const modal = useTemplateRef<InstanceType<typeof NewModal>>('modal')
+const search = ref('')
+const { formatMessage } = useVIntl()
+
+const messages = defineMessages({
+	ineligible: {
+		id: 'app.settings.synced-options.choose-sync-source.ineligible',
+		defaultMessage: 'You cannot choose this instance as a sync source',
+	},
+	search: {
+		id: 'app.settings.synced-options.choose-sync-source.search-placeholder',
+		defaultMessage: 'Search instance',
+	},
+	empty: {
+		id: 'app.settings.synced-options.choose-sync-source.no-instances-found',
+		defaultMessage: 'No instances found',
+	},
+	sync: {
+		id: 'app.settings.synced-options.choose-sync-source.sync',
+		defaultMessage: 'Sync',
+	},
+	loadError: {
+		id: 'app.settings.synced-options.choose-sync-source.load-error',
+		defaultMessage: 'Could not load sync sources. Please try again.',
+	},
+	retry: {
+		id: 'app.settings.synced-options.choose-sync-source.retry',
+		defaultMessage: 'Try again',
+	},
+})
+
+const filteredSources = computed(() => {
+	const query = search.value.trim().toLocaleLowerCase()
+	return props.sources.filter((source) => source.name.toLocaleLowerCase().includes(query))
+})
+const canConfirm = computed(
+	() =>
+		!props.loading &&
+		!props.pending &&
+		!props.error &&
+		props.sources.some((source) => source.id === selectedInstanceId.value && source.eligible),
+)
+
+function show() {
+	search.value = ''
+	modal.value?.show()
+}
+
+function hide() {
+	return modal.value?.hide()
+}
+
+defineExpose({ show, hide })
+</script>
+
+<template>
+	<NewModal
+		ref="modal"
+		:header="title"
+		no-padding
+		actions-divider
+		max-width="560px"
+		width="560px"
+		:disable-close="pending"
+		:on-after-hide="() => emit('close')"
+	>
+		<p
+			class="m-0 border-0 border-b border-solid border-surface-5 p-6 text-[var(--color-text-default)]"
+		>
+			{{ description }}
+		</p>
+
+		<div class="flex h-[400px] flex-col gap-3 overflow-y-auto bg-surface-2 px-6 py-4">
+			<StyledInput
+				v-model="search"
+				:icon="SearchIcon"
+				type="search"
+				autocomplete="off"
+				:placeholder="formatMessage(messages.search)"
+				:aria-label="formatMessage(messages.search)"
+				class="shrink-0"
+			/>
+
+			<div v-if="loading" class="flex flex-1 items-center justify-center" aria-busy="true">
+				<SpinnerIcon class="size-5 animate-spin text-[var(--color-text-tertiary)]" />
+			</div>
+			<div v-else-if="error" class="flex flex-1 flex-col items-center justify-center gap-3">
+				<p role="alert" class="m-0 text-center text-[var(--color-text-default)]">
+					{{ formatMessage(messages.loadError) }}
+				</p>
+				<Button @click="emit('retry')">{{ formatMessage(messages.retry) }}</Button>
+			</div>
+			<div
+				v-else-if="filteredSources.length === 0"
+				class="flex flex-1 items-center justify-center text-[var(--color-text-tertiary)]"
+			>
+				{{ formatMessage(messages.empty) }}
+			</div>
+			<div v-else role="radiogroup" :aria-label="title" class="flex flex-col gap-1">
+				<div
+					v-for="source in filteredSources"
+					:key="source.id"
+					v-tooltip="!source.eligible ? formatMessage(messages.ineligible) : undefined"
+				>
+					<CheckCircleButton
+						:checked="selectedInstanceId === source.id"
+						:disabled="pending || !source.eligible"
+						class="min-h-10"
+						@click="selectedInstanceId = source.id"
+					>
+						<span class="size-5 shrink-0 overflow-hidden rounded-[6px]">
+							<Avatar
+								:src="getInstanceIconUrl(source.icon_path)"
+								:alt="source.name"
+								:tint-by="source.id"
+								size="1.25rem"
+								no-shadow
+							/>
+						</span>
+						<span class="min-w-0 flex-1 truncate">{{ source.name }}</span>
+					</CheckCircleButton>
+				</div>
+			</div>
+		</div>
+		<template #actions>
+			<div class="flex justify-end gap-2 p-2">
+				<Button type="outlined" :disabled="pending" @click="hide">
+					<XIcon aria-hidden="true" />
+					{{ formatMessage(commonMessages.cancelButton) }}
+				</Button>
+				<Button
+					type="colored"
+					color="brand"
+					:disabled="!canConfirm"
+					:loading="pending"
+					@click="canConfirm && emit('confirm')"
+				>
+					<RefreshCwIcon :class="{ 'animate-spin': pending }" aria-hidden="true" />
+					{{ formatMessage(messages.sync) }}
+				</Button>
+			</div>
+		</template>
+	</NewModal>
+</template>