feat: 新增统一启动屏组件，优化跨平台加载体验

1. 新增splash-screen全局启动屏组件，适配非APP平台的加载过渡
2. 新增splash-state状态管理模块，统一处理启动屏的显示隐藏、软隐藏时序、兜底超时逻辑
3. 重构App.vue和main.js的加载逻辑，替换原有uni.showLoading方案
4. 在首页和空页面挂载启动屏组件，适配不同页面场景
5. 兼容H5平台降级处理，解决main.js挂载前无法渲染组件的问题

components/splash-screen/splash-screen.vue

@@ -0,0 +1,178 @@
+<template>
+	<!-- #ifndef APP-PLUS -->
+	<view v-if="visible" class="splash-screen" :class="{ 'splash-screen--leaving': leaving }">
+		<view class="splash-screen__bg"></view>
+		<view class="splash-screen__content">
+			<view class="splash-screen__logo">
+				<view class="splash-screen__logo-inner">S</view>
+			</view>
+			<view class="splash-screen__name">Shop</view>
+			<view class="splash-screen__loader">
+				<view class="splash-screen__dot"></view>
+				<view class="splash-screen__dot"></view>
+				<view class="splash-screen__dot"></view>
+			</view>
+		</view>
+	</view>
+	<!-- #endif -->
+</template>
+
+<script>
+	import splashState from '@/common/js/splash-state.js';
+
+	export default {
+		name: 'splash-screen',
+		data() {
+			return {
+				visible: false,
+				leaving: false
+			};
+		},
+		mounted() {
+			// 1) 启动时同步读取状态 (防止 mount 前已经 hide 的情况)
+			if (splashState.isShow()) {
+				this.visible = true;
+			}
+			// 2) 通知 state 模块: 组件已 mount
+			//    如果此时 splashState 有 _pendingHide (软隐藏), 会触发真关
+			splashState.notifyMounted();
+			// 3) 监听后续事件
+			splashState.on('splash:show', () => {
+				this.visible = true;
+				this.leaving = false;
+			});
+			splashState.on('splash:hide', () => {
+				this._leave();
+			});
+		},
+		beforeDestroy() {
+			splashState.off('splash:show');
+			splashState.off('splash:hide');
+			splashState.notifyUnmounted();
+			if (this._leaveTimer) {
+				clearTimeout(this._leaveTimer);
+				this._leaveTimer = null;
+			}
+		},
+		methods: {
+			_leave() {
+				// 触发淡出动画,动画结束再 unmount
+				this.leaving = true;
+				if (this._leaveTimer) clearTimeout(this._leaveTimer);
+				this._leaveTimer = setTimeout(() => {
+					this.visible = false;
+					this.leaving = false;
+				}, 320);
+			}
+		}
+	};
+</script>
+
+<style lang="scss" scoped>
+	.splash-screen {
+		position: fixed;
+		top: 0;
+		left: 0;
+		right: 0;
+		bottom: 0;
+		z-index: 99999;
+		display: flex;
+		align-items: center;
+		justify-content: center;
+		opacity: 1;
+		transition: opacity 0.3s ease;
+
+		&--leaving {
+			opacity: 0;
+		}
+	}
+
+	.splash-screen__bg {
+		position: absolute;
+		top: 0;
+		left: 0;
+		right: 0;
+		bottom: 0;
+		background: linear-gradient(180deg, #ffffff 0%, #f7f7f7 100%);
+	}
+
+	.splash-screen__content {
+		position: relative;
+		display: flex;
+		flex-direction: column;
+		align-items: center;
+		justify-content: center;
+	}
+
+	.splash-screen__logo {
+		width: 96rpx;
+		height: 96rpx;
+		border-radius: 24rpx;
+		background: #FF0036;
+		display: flex;
+		align-items: center;
+		justify-content: center;
+		box-shadow: 0 8rpx 24rpx rgba(255, 0, 54, 0.18);
+		animation: splash-pop 0.5s cubic-bezier(0.34, 1.56, 0.64, 1) both;
+	}
+
+	.splash-screen__logo-inner {
+		color: #fff;
+		font-size: 56rpx;
+		font-weight: 700;
+		font-family: -apple-system, BlinkMacSystemFont, 'Helvetica Neue', sans-serif;
+	}
+
+	.splash-screen__name {
+		margin-top: 28rpx;
+		font-size: 32rpx;
+		font-weight: 600;
+		color: #333;
+		letter-spacing: 1rpx;
+	}
+
+	.splash-screen__loader {
+		margin-top: 48rpx;
+		display: flex;
+		gap: 12rpx;
+	}
+
+	.splash-screen__dot {
+		width: 12rpx;
+		height: 12rpx;
+		border-radius: 50%;
+		background: #FF0036;
+		opacity: 0.4;
+		animation: splash-bounce 1.2s ease-in-out infinite;
+
+		&:nth-child(2) {
+			animation-delay: 0.15s;
+		}
+
+		&:nth-child(3) {
+			animation-delay: 0.3s;
+		}
+	}
+
+	@keyframes splash-pop {
+		0% {
+			transform: scale(0.6);
+			opacity: 0;
+		}
+		100% {
+			transform: scale(1);
+			opacity: 1;
+		}
+	}
+
+	@keyframes splash-bounce {
+		0%, 80%, 100% {
+			transform: scale(0.7);
+			opacity: 0.4;
+		}
+		40% {
+			transform: scale(1);
+			opacity: 1;
+		}
+	}
+</style>