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/components/common/DefaultFallback.tsx

@@ -0,0 +1,54 @@
+import { handleError } from '../../utils/error-handler'
+
+interface DefaultFallbackProps {
+  error: Error
+  onReset: () => void
+}
+
+export default function DefaultFallback({ error, onReset }: DefaultFallbackProps) {
+  const appError = handleError(error)
+
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-dark-900 p-4">
+      <div className="max-w-md w-full bg-dark-800 rounded-xl p-6 border border-dark-700">
+        <div className="flex items-center gap-3 mb-4">
+          <div className="w-10 h-10 bg-red-500/20 rounded-lg flex items-center justify-center">
+            <svg className="w-6 h-6 text-red-400" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z" />
+            </svg>
+          </div>
+          <div>
+            <h2 className="text-lg font-semibold text-gray-200">出错了</h2>
+            <p className="text-sm text-gray-500">应用遇到意外问题</p>
+          </div>
+        </div>
+
+        <div className="bg-dark-900 rounded-lg p-4 mb-4">
+          <p className="text-sm text-gray-300 mb-2">{appError.message}</p>
+          {appError.recoverable && (
+            <p className="text-xs text-green-400">此错误可以恢复，请尝试刷新页面</p>
+          )}
+        </div>
+
+        <div className="flex gap-3">
+          <button
+            onClick={onReset}
+            className="flex-1 px-4 py-2 bg-accent-blue hover:bg-blue-600 text-white rounded-lg transition-colors text-sm font-medium"
+          >
+            刷新页面
+          </button>
+          <button
+            onClick={() => window.location.reload()}
+            className="px-4 py-2 bg-dark-700 hover:bg-dark-600 text-gray-300 rounded-lg transition-colors text-sm"
+          >
+            重启应用
+          </button>
+        </div>
+
+        <p className="mt-4 text-xs text-gray-600 text-center">
+          如果问题持续存在，请检查日志或联系支持
+        </p>
+      </div>
+    </div>
+  )
+}