feat: 移植 Axolotl 皮肤盔甲预览（Armor Preview）

从 Axolotl 1.9.7 移植皮肤编辑器的盔甲/盔甲纹饰预览功能，
让玩家在皮肤选择页可试穿盔甲并调整纹饰。

新增：
- 前端组件 ArmorTrimTab / SkinListSkeleton 及 skin-armor-preview helper
- packages/ui 组件 ArmorPreviewControls / ArmorTrimPanel
- packages/ui armor 预览 composable（armor-preview-assets/types、use-armor-preview）
- packages/ui webgl 盔甲渲染（armor-geometry / armor-preview-object / armor-rendering）
- packages/ui/src/assets/minecraft-armor 盔甲素材

改动：
- SkinPreviewRenderer 增加 armorPreview/armorControls props、
  armorConfig model 与 useArmorPreview 集成
- Skins.vue 增加盔甲 tab、状态逻辑与模板接入
- 补齐 skin/composables/utils 的导出

适配说明：ArmorPreviewControls 原版依赖 reka-ui 的 popover，
Starlight 无该依赖且只用 external 模式，故改写为 external-only，
去除 reka-ui 与 overlays.css 依赖。

apps/app-frontend/src/components/ui/skin/ArmorTrimTab.vue

@@ -0,0 +1,123 @@
+<script setup lang="ts">
+import { CheckIcon, RotateCounterClockwiseIcon, XIcon } from '@modrinth/assets'
+import {
+	type ArmorPreviewConfig,
+	ArmorTrimPanel,
+	Button,
+	defineMessages,
+	useVIntl,
+} from '@modrinth/ui'
+import { computed, onMounted, onUnmounted } from 'vue'
+
+const messages = defineMessages({
+	title: { id: 'app.skins.armor.title', defaultMessage: 'Armour and trims' },
+	description: {
+		id: 'app.skins.armor.description',
+		defaultMessage: 'Try armour and trims on your skin. Nothing is stored until you save it.',
+	},
+	unsaved: { id: 'app.skins.armor.unsaved', defaultMessage: 'Unsaved changes' },
+	resetToSaved: { id: 'app.skins.armor.reset-to-saved', defaultMessage: 'Reset to saved' },
+	restoreDefaults: {
+		id: 'app.skins.armor.restore-defaults',
+		defaultMessage: 'Restore defaults',
+	},
+	save: { id: 'app.skins.armor.save', defaultMessage: 'Save' },
+	close: { id: 'app.skins.armor.close', defaultMessage: 'Close' },
+})
+
+const draft = defineModel<ArmorPreviewConfig>({ required: true })
+const props = defineProps<{
+	saved: ArmorPreviewConfig
+	/** Stable id shared with the toggle so it can point `aria-controls` here. */
+	panelId: string
+}>()
+const emit = defineEmits<{
+	save: []
+	reset: []
+	defaults: []
+	close: []
+}>()
+
+const { formatMessage } = useVIntl()
+
+/**
+ * The tab is mounted only while it is showing, so this listens exactly for that
+ * window. It listens in the capture phase because the app's own global
+ * shortcuts listen on `window` the same way, and a bubble-phase listener here
+ * would be skipped as soon as anything closer stopped the event. A modal on top
+ * of the page owns Escape while it is up, and focus sits inside it, so the key
+ * is left to it.
+ */
+function handleKeydown(event: KeyboardEvent) {
+	if (event.key !== 'Escape') return
+	if (
+		event.target instanceof Element &&
+		event.target.closest('[role="dialog"], [role="alertdialog"], [aria-modal="true"]')
+	) {
+		return
+	}
+
+	emit('close')
+}
+
+onMounted(() => window.addEventListener('keydown', handleKeydown, true))
+onUnmounted(() => window.removeEventListener('keydown', handleKeydown, true))
+
+const isDirty = computed(() =>
+	Object.entries(draft.value).some(([slot, piece]) => {
+		const applied = props.saved[slot as keyof ArmorPreviewConfig]
+		return (
+			!applied ||
+			applied.material !== piece.material ||
+			applied.trimPattern !== piece.trimPattern ||
+			applied.trimMaterial !== piece.trimMaterial
+		)
+	}),
+)
+</script>
+
+<template>
+	<div
+		:id="panelId"
+		role="region"
+		:aria-label="formatMessage(messages.title)"
+		class="flex min-h-[24rem] flex-col gap-4 rounded-2xl border border-solid border-surface-5 bg-surface-2 p-4"
+	>
+		<header class="flex flex-wrap items-start justify-between gap-3">
+			<div class="min-w-0">
+				<h2 class="m-0 text-lg font-bold text-[var(--color-text-primary)]">
+					{{ formatMessage(messages.title) }}
+				</h2>
+				<p class="m-0 mt-1 text-sm leading-6 text-[var(--color-text-tertiary)]">
+					{{ formatMessage(messages.description) }}
+				</p>
+			</div>
+			<div class="flex shrink-0 flex-wrap items-center gap-2">
+				<span v-if="isDirty" class="text-sm font-semibold text-[var(--color-text-tertiary)]">
+					{{ formatMessage(messages.unsaved) }}
+				</span>
+				<Button type="outlined" @click="emit('close')"
+					><XIcon />{{ formatMessage(messages.close) }}</Button
+				>
+			</div>
+		</header>
+
+		<ArmorTrimPanel v-model="draft" />
+
+		<footer
+			class="mt-auto flex flex-wrap items-center justify-between gap-2 border-0 border-t border-solid border-surface-5 pt-3"
+		>
+			<div class="flex flex-wrap items-center gap-2">
+				<Button type="outlined" :disabled="!isDirty" @click="emit('reset')"
+					><RotateCounterClockwiseIcon />{{ formatMessage(messages.resetToSaved) }}</Button
+				>
+				<Button type="outlined" @click="emit('defaults')">{{
+					formatMessage(messages.restoreDefaults)
+				}}</Button>
+			</div>
+			<Button type="colored" color="brand" :disabled="!isDirty" @click="emit('save')"
+				><CheckIcon />{{ formatMessage(messages.save) }}</Button
+			>
+		</footer>
+	</div>
+</template>