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/helpers/skin-armor-preview.test.ts

@@ -0,0 +1,60 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+
+import { loadSkinArmorPreview, saveSkinArmorPreview } from './skin-armor-preview.ts'
+
+const originalStorage = Object.getOwnPropertyDescriptor(globalThis, 'localStorage')
+
+function withStorage(run: (values: Map<string, string>) => void): void {
+	const values = new Map<string, string>()
+	Object.defineProperty(globalThis, 'localStorage', {
+		configurable: true,
+		value: {
+			getItem: (key: string) => values.get(key) ?? null,
+			setItem: (key: string, value: string) => values.set(key, value),
+		},
+	})
+	try {
+		run(values)
+	} finally {
+		if (originalStorage) Object.defineProperty(globalThis, 'localStorage', originalStorage)
+		else delete (globalThis as { localStorage?: Storage }).localStorage
+	}
+}
+
+test('armor preview selections survive storage round trips', () => {
+	withStorage(() => {
+		const config = loadSkinArmorPreview()
+		config.helmet.material = 'diamond'
+		config.helmet.trimPattern = 'sentry'
+		config.helmet.trimMaterial = 'gold'
+		saveSkinArmorPreview(config)
+
+		assert.deepEqual(loadSkinArmorPreview(), config)
+	})
+})
+
+test('invalid stored armor values fall back per slot and field', () => {
+	withStorage((values) => {
+		values.set(
+			'axolotl:skin-armor-preview',
+			JSON.stringify({
+				helmet: { material: 'diamond', trimPattern: 'invalid', trimMaterial: 'gold' },
+				boots: { material: 'turtle', trimPattern: 'sentry', trimMaterial: 'invalid' },
+			}),
+		)
+
+		const config = loadSkinArmorPreview()
+		assert.deepEqual(config.helmet, {
+			material: 'diamond',
+			trimPattern: null,
+			trimMaterial: 'gold',
+		})
+		assert.deepEqual(config.boots, {
+			material: null,
+			trimPattern: 'sentry',
+			trimMaterial: 'iron',
+		})
+		assert.equal(config.chestplate.material, null)
+	})
+})