feat(player): 新增长页面一键回到顶部按钮

- 在 MainLayout 主滚动区挂载浮动回顶按钮，投注列表与赛事详情等长页通用

- 滚动超过阈值后显示，支持平滑回顶与底部导航避让

- 补充中/英/马来语文案 common.back_to_top

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/player/src/components/BackToTopButton.vue

@@ -0,0 +1,79 @@
+<script setup lang="ts">
+import { toRef } from 'vue';
+import { useI18n } from 'vue-i18n';
+import { useBackToTop } from '../composables/useBackToTop';
+
+const props = defineProps<{
+  scrollEl: HTMLElement | null;
+  aboveNav?: boolean;
+}>();
+
+const { t } = useI18n();
+const { visible, scrollToTop } = useBackToTop(toRef(props, 'scrollEl'));
+</script>
+
+<template>
+  <Transition name="back-top-fade">
+    <button
+      v-if="visible"
+      type="button"
+      class="back-top-btn"
+      :class="{ 'above-nav': aboveNav }"
+      :aria-label="t('common.back_to_top')"
+      @click="scrollToTop"
+    >
+      <svg viewBox="0 0 24 24" aria-hidden="true">
+        <path
+          d="M12 5.5 6 11.5l1.4 1.4 3.6-3.6V18h2V9.3l3.6 3.6 1.4-1.4-6-6Z"
+          fill="currentColor"
+        />
+      </svg>
+    </button>
+  </Transition>
+</template>
+
+<style scoped>
+.back-top-btn {
+  position: fixed;
+  right: 16px;
+  bottom: calc(12px + env(safe-area-inset-bottom, 0px));
+  z-index: 120;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 44px;
+  height: 44px;
+  border-radius: 50%;
+  border: 1px solid var(--border-gold-soft, rgba(200, 168, 78, 0.35));
+  background: rgba(17, 17, 17, 0.92);
+  color: var(--primary-light, #c8a84e);
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.45);
+  cursor: pointer;
+  transition: background 0.2s, transform 0.2s;
+}
+
+.back-top-btn.above-nav {
+  bottom: calc(68px + env(safe-area-inset-bottom, 0px));
+}
+
+.back-top-btn:active {
+  background: rgba(200, 168, 78, 0.16);
+  transform: scale(0.96);
+}
+
+.back-top-btn svg {
+  width: 22px;
+  height: 22px;
+}
+
+.back-top-fade-enter-active,
+.back-top-fade-leave-active {
+  transition: opacity 0.2s ease, transform 0.2s ease;
+}
+
+.back-top-fade-enter-from,
+.back-top-fade-leave-to {
+  opacity: 0;
+  transform: translateY(8px);
+}
+</style>