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/utils/privacy-mode.js

@@ -0,0 +1,20 @@
+export const PRIVACY_MODE_KEY = 'ui.privacy_mode'
+
+export const readPrivacyMode = (fallback = false) => {
+  if (!process.client) return !!fallback
+  try {
+    const raw = localStorage.getItem(PRIVACY_MODE_KEY)
+    if (raw == null) return !!fallback
+    const normalized = String(raw).trim().toLowerCase()
+    return normalized === '1' || normalized === 'true'
+  } catch {
+    return !!fallback
+  }
+}
+
+export const writePrivacyMode = (enabled) => {
+  if (!process.client) return
+  try {
+    localStorage.setItem(PRIVACY_MODE_KEY, enabled ? '1' : '0')
+  } catch {}
+}