feat: add comprehensive backend topics and fix build issues

## 新增内容

### 附录文档扩展
- 扩展前端项目架构文档 (frontend-project-architecture.md)
- 扩展后端项目架构文档 (backend-project-architecture.md)
- 扩展数据治理文档 (data-governance.md)
- 扩展数据可视化文档 (data-visualization.md)
- 扩展分布式系统文档 (distributed-systems.md)
- 扩展高可用文档 (high-availability.md)
- 扩展单体到微服务文档 (monolith-to-microservices.md)
- 扩展系统设计方法论文档 (system-design-methodology.md)
- 扩展 Docker 容器文档 (docker-containers.md)
- 扩展 Kubernetes 文档 (kubernetes.md)
- 扩展 Linux 基础文档 (linux-basics.md)
- 扩展神经网络文档 (neural-networks.md)

### 新增交互式组件
- 数据治理组件: DataQualityDemo, DataGovernanceFrameworkDemo, DataLineageDemo
- 数据可视化组件: ChartTypeSelectorDemo, DashboardLayoutDemo
- 分布式系统组件: CAPTheoremDemo, ConsistencyModelsDemo, DistributedChallengesDemo
- 高可用组件: AvailabilityCalculatorDemo, FailoverStrategyDemo
- 系统设计组件: SystemDesignStepsDemo, CapacityEstimationDemo
- Docker 容器组件: DockerArchitectureDemo, DockerLifecycleDemo
- Kubernetes 组件: K8sArchitectureDemo, K8sWorkloadsDemo
- Linux 基础组件: LinuxFileSystemDemo, LinuxCommandDemo, LinuxPermissionsDemo
- 神经网络组件: NeuronDemo, NetworkLayersDemo, NetworkArchitectureDemo
- 单体到微服务组件: ArchEvolutionDemo
- 项目架构组件: ProjectArchitectureComparisonDemo
- 附录导航组件: AppendixFlowMap

### 英文版重构
- 将 en-us 目录重命名为 en
- 更新相关配置和组件中的语言代码

## Bug 修复
- 修复 index.js 中重复的组件导入语句导致的 build 失败
- 恢复被注释的 InvertedIndexDemo 和 SearchRelevanceDemo 导入
- 修复 HomeFeatures.vue 中 en-us 与 config.mjs 中 en 不一致导致的语言切换问题

## 其他改进
- 添加构建脚本 (scripts/build.mjs)
- 更新依赖版本

docs/.vitepress/theme/components/appendix/high-availability/AvailabilityCalculatorDemo.vue

@@ -0,0 +1,87 @@
+<!--
+  AvailabilityCalculatorDemo.vue
+  可用性计算器：展示不同 SLA 级别对应的停机时间
+-->
+<template>
+  <div class="availability-demo">
+    <div class="header">
+      <div class="title">可用性等级计算器</div>
+      <div class="subtitle">点击查看不同"几个 9"对应的停机时间</div>
+    </div>
+
+    <div class="sla-cards">
+      <div
+        v-for="sla in slaLevels"
+        :key="sla.nines"
+        :class="['sla-card', { active: activeSla === sla.nines }]"
+        @click="activeSla = sla.nines"
+      >
+        <div class="sla-nines">{{ sla.label }}</div>
+        <div class="sla-percent">{{ sla.percent }}</div>
+      </div>
+    </div>
+
+    <div v-if="current" class="detail-panel">
+      <div class="detail-title">{{ current.label }}（{{ current.percent }}）</div>
+      <div class="downtime-grid">
+        <div class="downtime-item">
+          <div class="dt-label">每年停机</div>
+          <div class="dt-value">{{ current.yearly }}</div>
+        </div>
+        <div class="downtime-item">
+          <div class="dt-label">每月停机</div>
+          <div class="dt-value">{{ current.monthly }}</div>
+        </div>
+        <div class="downtime-item">
+          <div class="dt-label">每周停机</div>
+          <div class="dt-value">{{ current.weekly }}</div>
+        </div>
+      </div>
+      <div class="detail-examples">
+        <span class="label">典型场景：</span>{{ current.examples }}
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { ref, computed } from 'vue'
+
+const activeSla = ref('3')
+
+const slaLevels = [
+  { nines: '2', label: '2 个 9', percent: '99%', yearly: '3.65 天', monthly: '7.3 小时', weekly: '1.68 小时', examples: '内部工具、非关键系统' },
+  { nines: '3', label: '3 个 9', percent: '99.9%', yearly: '8.76 小时', monthly: '43.8 分钟', weekly: '10.1 分钟', examples: '普通 Web 应用、企业系统' },
+  { nines: '4', label: '4 个 9', percent: '99.99%', yearly: '52.6 分钟', monthly: '4.38 分钟', weekly: '1.01 分钟', examples: '电商平台、SaaS 服务' },
+  { nines: '5', label: '5 个 9', percent: '99.999%', yearly: '5.26 分钟', monthly: '26.3 秒', weekly: '6.05 秒', examples: '金融交易、电信核心网' }
+]
+
+const current = computed(() => slaLevels.find(s => s.nines === activeSla.value))
+</script>
+
+<style scoped>
+.availability-demo {
+  border: 1px solid var(--vp-c-divider); background: var(--vp-c-bg-soft);
+  border-radius: 12px; padding: 1.5rem; margin: 1.5rem 0;
+}
+.header { margin-bottom: 1rem; }
+.title { font-weight: 700; font-size: 1.1rem; }
+.subtitle { color: var(--vp-c-text-2); font-size: 0.9rem; }
+.sla-cards { display: grid; grid-template-columns: repeat(auto-fit, minmax(100px, 1fr)); gap: 0.5rem; margin-bottom: 1rem; }
+.sla-card {
+  padding: 0.6rem; border-radius: 8px; cursor: pointer; text-align: center;
+  background: var(--vp-c-bg); border: 2px solid var(--vp-c-divider); transition: all 0.2s;
+}
+.sla-card:hover { border-color: var(--vp-c-brand); }
+.sla-card.active { border-color: var(--vp-c-brand); background: rgba(var(--vp-c-brand-rgb), 0.08); }
+.sla-nines { font-weight: 800; font-size: 1.1rem; color: var(--vp-c-brand); }
+.sla-percent { font-size: 0.8rem; color: var(--vp-c-text-2); }
+.detail-panel { background: var(--vp-c-bg); border-radius: 8px; padding: 1rem; border: 1px solid var(--vp-c-divider); }
+.detail-title { font-weight: 700; font-size: 0.95rem; margin-bottom: 0.75rem; }
+.downtime-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: 0.5rem; margin-bottom: 0.75rem; }
+.downtime-item { text-align: center; padding: 0.5rem; border-radius: 6px; background: var(--vp-c-bg-soft); }
+.dt-label { font-size: 0.75rem; color: var(--vp-c-text-3); }
+.dt-value { font-weight: 700; font-size: 0.9rem; color: var(--vp-c-brand); }
+.detail-examples { font-size: 0.82rem; }
+.label { font-weight: 600; color: var(--vp-c-text-2); }
+</style>