refactor(chat-ui): 抽离侧边栏并统一账号/实时/隐私状态

新增 SidebarRail 组件并统一主导航入口

引入 chatAccounts/chatRealtime/privacy 三个 Pinia store 复用全局状态

聊天/联系人/朋友圈页面去重侧栏逻辑，app 根布局统一承载标题栏与内容区

frontend/stores/privacy.js

@@ -0,0 +1,31 @@
+import { defineStore } from 'pinia'
+
+import { readPrivacyMode, writePrivacyMode } from '~/utils/privacy-mode'
+
+export const usePrivacyStore = defineStore('privacy', () => {
+  const privacyMode = ref(false)
+  const initialized = ref(false)
+
+  const init = () => {
+    if (initialized.value) return
+    initialized.value = true
+    privacyMode.value = readPrivacyMode(false)
+  }
+
+  const set = (enabled) => {
+    privacyMode.value = !!enabled
+    writePrivacyMode(privacyMode.value)
+  }
+
+  const toggle = () => {
+    set(!privacyMode.value)
+  }
+
+  return {
+    privacyMode,
+    init,
+    set,
+    toggle,
+  }
+})
+