feat(frontend): 移动端底部导航与 Hero 插画及页脚浅色化

- MobileBottomNav.vue(新增): 三 Tab 底部导航栏（点歌/排期/我的）
  lg 1024px 以上隐藏；iOS safe-area 兼容；点击平滑滚动或跳转 /account
- HeroIllustration.vue(新增): 纯 SVG 广播园艺六花瓣+麦克风插画
  紫粉蓝三色渐变花瓣+花蕊麦克风+三层虚线装饰环；viewBox 400×400
- SiteFooter.vue: 改浅色适配（白底+薄边+紫强调色 hover）
- index.vue: Hero 插图槽替换为 HeroIllustration；集成 MobileBottomNav；
  新增 handleMobileNav 响应底部导航点击事件

app/components/UI/MobileBottomNav.vue

@@ -0,0 +1,40 @@
+<template>
+  <nav
+    class="fixed bottom-0 left-0 right-0 z-40 lg:hidden bg-white border-t border-gray-200 px-4 py-2 pb-[max(8px,env(safe-area-inset-bottom))] backdrop-blur-sm shadow-[0_-2px_16px_rgba(0,0,0,0.04)]"
+  >
+    <div class="grid grid-cols-3 gap-2">
+      <button
+        type="button"
+        class="flex flex-col items-center justify-center gap-1 py-1 text-gray-500"
+        @click="$emit('navigate', 'request-section')"
+      >
+        <Music :size="20" />
+        <span class="text-[11px] font-semibold">点歌</span>
+      </button>
+
+      <button
+        type="button"
+        class="flex flex-col items-center justify-center gap-1 py-1 text-gray-500"
+        @click="$emit('navigate', 'schedule-section')"
+      >
+        <CalendarDays :size="20" />
+        <span class="text-[11px] font-semibold">排期</span>
+      </button>
+
+      <button
+        type="button"
+        class="flex flex-col items-center justify-center gap-1 py-1 text-gray-500"
+        @click="$emit('navigate', 'account')"
+      >
+        <User :size="20" />
+        <span class="text-[11px] font-semibold">我的</span>
+      </button>
+    </div>
+  </nav>
+</template>
+
+<script setup>
+import { Music, CalendarDays, User } from '@lucide/vue'
+
+defineEmits(['navigate'])
+</script>