feat: 新增多语言README并优化文档结构

新增德语、俄语、韩语、繁体中文多语言README，英文与简体中文README同步优化，统一下载链接与徽章样式，完善多语言入口。提升国际化支持与文档可读性。

src/components/common/BackToTop.vue

@@ -0,0 +1,48 @@
+<template>
+  <Transition
+    enter-active-class="transition-all duration-300 ease-out"
+    enter-from-class="translate-y-4 opacity-0"
+    enter-to-class="translate-y-0 opacity-100"
+    leave-active-class="transition-all duration-200 ease-in"
+    leave-from-class="translate-y-0 opacity-100"
+    leave-to-class="translate-y-4 opacity-0"
+  >
+    <Button v-if="isVisible" variant="outline" size="icon" @click="scrollToTop">
+      <ChevronUp class="h-4 w-4" />
+    </Button>
+  </Transition>
+</template>
+
+<script setup lang="ts">
+  import { ref, onMounted, onUnmounted } from 'vue'
+  import { Button } from '@/components/ui/button'
+  import { ChevronUp } from 'lucide-vue-next'
+
+  // 显示阈值（滚动超过这个距离才显示按钮）
+  const SCROLL_THRESHOLD = 300
+
+  const isVisible = ref(false)
+
+  // 监听滚动事件
+  const handleScroll = () => {
+    isVisible.value = window.scrollY > SCROLL_THRESHOLD
+  }
+
+  // 丝滑返回顶部
+  const scrollToTop = () => {
+    window.scrollTo({
+      top: 0,
+      behavior: 'smooth'
+    })
+  }
+
+  onMounted(() => {
+    window.addEventListener('scroll', handleScroll, { passive: true })
+    // 初始检查
+    handleScroll()
+  })
+
+  onUnmounted(() => {
+    window.removeEventListener('scroll', handleScroll)
+  })
+</script>