refactor: 全面重构前端 UI 样式

- 重构 theme.css：替换刺眼青色背景为柔和灰蓝(#eef1f6)，品牌色改为靛蓝(#5b6ef5)
- 统一按钮体系：主按钮36px固定高度，菜单按钮30px，表格按钮28px，尺寸协调
- 登录页：服务器地址与端口并排布局，输入框40px高度统一，卡片阴影加深
- 面板页：导航栏56px高度，内容区居中限宽1400px，滚动条美化
- 概览页：auto-fit网格+340px最小宽度防止卡片不均，进程表全宽跨列
- 进程表格：table-layout:fixed + 列宽分配 + nowrap 防止串行溢出
- 所有文字元素加 white-space:nowrap 防止换行串行
- 更新 Electron 窗口背景色匹配新主题

src/main/index.js

@@ -46,7 +46,7 @@ function createWindow() {
   mainWindow = new BrowserWindow({
     width: 1280,
     height: 800,
-    backgroundColor: '#d2f8fc',
+    backgroundColor: '#eef1f6',
     webPreferences: {
       preload: path.join(__dirname, '../preload/index.js'),
       sandbox: false,
@@ -121,6 +121,49 @@ ipcMain.handle('ssh:disconnect', () => {
 
 ipcMain.handle('ssh:session', () => activeSession)
 
+// 在活动的 SSH 连接上执行命令
+function execSsh(cmd, timeoutMs = 10000) {
+  return new Promise((resolve) => {
+    if (!activeConnection) {
+      resolve({ ok: false, message: 'SSH 连接不存在' })
+      return
+    }
+
+    let settled = false
+    let stdout = ''
+    let stderr = ''
+
+    const timer = setTimeout(() => {
+      if (!settled) {
+        settled = true
+        resolve({ ok: false, message: '命令执行超时' })
+      }
+    }, timeoutMs)
+
+    activeConnection.exec(cmd, (err, stream) => {
+      if (err) {
+        clearTimeout(timer)
+        if (!settled) {
+          settled = true
+          resolve({ ok: false, message: err.message })
+        }
+        return
+      }
+      stream.on('data', (chunk) => (stdout += chunk.toString()))
+      stream.stderr.on('data', (chunk) => (stderr += chunk.toString()))
+      stream.on('close', (code) => {
+        clearTimeout(timer)
+        if (!settled) {
+          settled = true
+          resolve({ ok: true, code, stdout, stderr })
+        }
+      })
+    })
+  })
+}
+
+ipcMain.handle('ssh:exec', (event, { cmd, timeout }) => execSsh(cmd, timeout))
+
 // ---------- IPC：凭据 ----------
 ipcMain.handle('cred:save', (event, { host, port, username, password }) => {
   const list = readCredentials()