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.ts

@@ -0,0 +1,47 @@
+import {
+	ARMOR_SLOTS,
+	ARMOR_TRIM_MATERIALS,
+	ARMOR_TRIM_PATTERNS,
+	armorMaterialsForSlot,
+	type ArmorPreviewConfig,
+	createDefaultArmorPreviewConfig,
+} from '@modrinth/ui'
+
+const STORAGE_KEY = 'axolotl:skin-armor-preview'
+
+function includes<T extends string>(values: readonly T[], value: unknown): value is T {
+	return typeof value === 'string' && values.some((item) => item === value)
+}
+
+export function loadSkinArmorPreview(): ArmorPreviewConfig {
+	const config = createDefaultArmorPreviewConfig()
+	try {
+		const saved = JSON.parse(globalThis.localStorage?.getItem(STORAGE_KEY) ?? 'null')
+		if (!saved || typeof saved !== 'object' || Array.isArray(saved)) return config
+
+		for (const slot of ARMOR_SLOTS) {
+			const piece = saved[slot]
+			if (!piece || typeof piece !== 'object' || Array.isArray(piece)) continue
+			if (piece.material === null || includes(armorMaterialsForSlot(slot), piece.material)) {
+				config[slot].material = piece.material
+			}
+			if (piece.trimPattern === null || includes(ARMOR_TRIM_PATTERNS, piece.trimPattern)) {
+				config[slot].trimPattern = piece.trimPattern
+			}
+			if (includes(ARMOR_TRIM_MATERIALS, piece.trimMaterial)) {
+				config[slot].trimMaterial = piece.trimMaterial
+			}
+		}
+	} catch {
+		// Invalid or unavailable local storage falls back to a fresh preview.
+	}
+	return config
+}
+
+export function saveSkinArmorPreview(config: ArmorPreviewConfig): void {
+	try {
+		globalThis.localStorage?.setItem(STORAGE_KEY, JSON.stringify(config))
+	} catch {
+		// The preview remains usable when local storage is unavailable.
+	}
+}