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/index.css

@@ -17,16 +17,36 @@
   --color-dark-700: #334155;
   --color-dark-600: #475569;
 
+  /* Semantic tokens (dark default) */
+  --color-surface-base: #0f172a;
+  --color-surface-raised: #1e293b;
+  --color-surface-overlay: #334155;
+  --color-surface-inset: #475569;
+
   --color-text-primary:   #e2e8f0;
   --color-text-secondary: #cbd5e1;
   --color-text-muted:     #94a3b8;
   --color-text-faint:     #64748b;
   --color-text-dim:       #475569;
 
+  --color-content-primary: #e2e8f0;
+  --color-content-secondary: #cbd5e1;
+  --color-content-tertiary: #94a3b8;
+  --color-content-inverse: #0f172a;
+
+  --color-border: #334155;
+  --color-border-strong: #475569;
+
   --color-accent-blue:   #3b82f6;
   --color-accent-green:  #10b981;
   --color-accent-purple: #8b5cf6;
   --color-accent-orange: #f97316;
+  --color-accent-red:    #ef4444;
+
+  --color-feedback-success: #10b981;
+  --color-feedback-warning: #f59e0b;
+  --color-feedback-error: #ef4444;
+  --color-feedback-info: #3b82f6;
 
   --color-scrollbar-track:   #1e293b;
   --color-scrollbar-thumb:   #475569;
@@ -35,45 +55,20 @@
   --color-terminal-bg: #0f172a;
 }
 
-/* ---- Light theme ---- */
-:root[data-theme='light'] {
-  --color-dark-900: #ffffff;
-  --color-dark-800: #f3f4f6;
-  --color-dark-700: #e5e7eb;
-  --color-dark-600: #d1d5db;
-
-  --color-text-primary:   #1f2937;
-  --color-text-secondary: #374151;
-  --color-text-muted:     #6b7280;
-  --color-text-faint:     #9ca3af;
-  --color-text-dim:       #d1d5db;
-
-  --color-accent-blue:   #2563eb;
-  --color-accent-green:  #059669;
-  --color-accent-purple: #7c3aed;
-  --color-accent-orange: #ea580c;
-
-  --color-scrollbar-track:   #f3f4f6;
-  --color-scrollbar-thumb:   #d1d5db;
-  --color-scrollbar-thumb-h: #9ca3af;
-
-  --color-terminal-bg: #ffffff;
-}
-
 * {
   margin: 0;
   padding: 0;
   box-sizing: border-box;
 }
 
-body {
+html, body {
   font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',
     'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue',
     sans-serif;
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
-  background-color: var(--color-dark-900);
-  color: var(--color-text-primary);
+  background-color: #0f172a !important;
+  color: #e2e8f0;
   overflow: hidden;
 }
 
@@ -82,6 +77,15 @@ body {
   height: 100vh;
 }
 
+/* Window chrome utilities */
+.drag {
+  -webkit-app-region: drag;
+}
+
+.no-drag {
+  -webkit-app-region: no-drag;
+}
+
 /* Custom scrollbar */
 ::-webkit-scrollbar {
   width: 8px;