v2.0.0: 主题系统+SSH私钥解析修复+会话持久化+连接池优化+UI打磨

核心修复:
- ssh-service.ts: 新增 resolvePrivateKey()，将私钥文件路径(~/.ssh/id_rsa)解析为内容后传给 ssh2，
  解决 'Cannot parse privateKey: Unsupported key format' 错误
- useTerminalConnection.ts + sessionHelper.ts: 根据 authType 过滤字段，password/agent 认证不再传 privateKey
- sessionStore.ts: addSession() 现在先调用 sessionCreate() 持久化到 SQLite，解决恢复标签'无可用连接信息'问题
- database-service.ts: updateSession() 补充 private_key_path 字段持久化
- SessionDialog.tsx: 默认 username='root'、privateKey='~/.ssh/id_rsa'

UI/UX:
- 语义化主题系统(app-theme.ts)，强制深色模式，全局无边框设计
- macOS 标题栏支持双击最大化(window:toggleMaximize IPC)
- macOS 菜单栏('OpsXShell' 应用菜单 + Settings Cmd+,)
- WelcomeView 4卡片布局，移除 Settings 按钮
- TabBar/TabItem 背景色统一(bg-dark-800/900)，消除白色边框线

架构优化:
- SFTP 生命周期修复(useSFTP.ts useRef + autoConnectAttempted guard)
- 连接池竞态修复，支持快速多标签连接
- IPC 接口规范化，安全错误处理
- Vite 代码分割优化

质量: lint零错误 | tsc零错误 | build成功 | 49 tests pass

src/hooks/useTheme.ts

@@ -6,10 +6,9 @@ import { darkColors, lightColors, type AppThemeMode, type ColorMap } from '../th
  * Resolves the *effective* theme ('dark' | 'light') from the user preference,
  * including the 'system' → prefers-color-scheme mapping.
  */
-function resolveEffectiveTheme(preference: AppThemeMode): 'dark' | 'light' {
-  if (preference !== 'system') return preference
-  if (typeof window === 'undefined') return 'dark'
-  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+function resolveEffectiveTheme(_preference: AppThemeMode): 'dark' | 'light' {
+  // 当前版本固定使用 dark 主题，后续版本开放主题切换
+  return 'dark'
 }
 
 /**
@@ -32,26 +31,28 @@ function applyCSSVars(colors: ColorMap) {
  */
 export function useTheme() {
   const theme = useSettingsStore((s) => s.settings.theme)
-  const isLoaded = useSettingsStore((s) => s.isLoaded)
 
   const effective = useMemo(() => resolveEffectiveTheme(theme), [theme])
 
   // Apply theme to DOM whenever effective theme changes
   useEffect(() => {
-    if (!isLoaded) return
-
     const colors = effective === 'dark' ? darkColors : lightColors
 
     // Set data attribute for potential direct CSS selectors
     document.documentElement.setAttribute('data-theme', effective)
+    if (effective === 'dark') {
+      document.documentElement.classList.add('dark')
+    } else {
+      document.documentElement.classList.remove('dark')
+    }
 
     // Apply CSS custom properties
     applyCSSVars(colors)
 
     // Update body bg & colour (mirrors index.css body rules but via vars)
-    document.body.style.backgroundColor = 'var(--color-dark-900)'
-    document.body.style.color = 'var(--color-text-primary)'
-  }, [effective, isLoaded])
+    document.body.style.backgroundColor = '#0f172a'
+    document.body.style.color = '#e2e8f0'
+  }, [effective])
 
   // Listen for system colour-scheme changes when preference is 'system'
   useEffect(() => {
@@ -64,8 +65,8 @@ export function useTheme() {
       const colors = e.matches ? darkColors : lightColors
       document.documentElement.setAttribute('data-theme', newEffective)
       applyCSSVars(colors)
-      document.body.style.backgroundColor = 'var(--color-dark-900)'
-      document.body.style.color = 'var(--color-text-primary)'
+      document.body.style.backgroundColor = '#0f172a'
+      document.body.style.color = '#e2e8f0'
     }
 
     mql.addEventListener('change', handler)