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/composables/useBackToTop.ts

@@ -0,0 +1,38 @@
+import { onMounted, onUnmounted, ref, watch, type Ref } from 'vue';
+
+export function useBackToTop(
+  scrollEl: Ref<HTMLElement | null>,
+  options?: { threshold?: number },
+) {
+  const visible = ref(false);
+  const threshold = options?.threshold ?? 320;
+
+  function onScroll() {
+    const el = scrollEl.value;
+    visible.value = !!el && el.scrollTop > threshold;
+  }
+
+  function scrollToTop() {
+    scrollEl.value?.scrollTo({ top: 0, behavior: 'smooth' });
+  }
+
+  function bind(el: HTMLElement | null) {
+    el?.addEventListener('scroll', onScroll, { passive: true });
+    onScroll();
+  }
+
+  function unbind(el: HTMLElement | null) {
+    el?.removeEventListener('scroll', onScroll);
+  }
+
+  onMounted(() => bind(scrollEl.value));
+
+  onUnmounted(() => unbind(scrollEl.value));
+
+  watch(scrollEl, (el, prev) => {
+    unbind(prev);
+    bind(el);
+  });
+
+  return { visible, scrollToTop, onScroll };
+}