feat(mcp): unify preset button styles with emerald theme

- Update MCP preset selector to match provider form layout
- Change button size from xs to sm (px-4 py-2) for consistency
- Use emerald-500/600 for selected state (MCP brand color)
- Add preset description text below buttons
- Fix cancel button hover effect in footer (gray-200/gray-700)
- Fix missing space in zh i18n: "应用到 Claude Code 插件"

src/i18n/locales/zh.json

@@ -103,7 +103,7 @@
     "windowBehavior": "窗口行为",
     "minimizeToTray": "关闭时最小化到托盘",
     "minimizeToTrayDescription": "勾选后点击关闭按钮会隐藏到系统托盘，取消则直接退出应用。",
-    "enableClaudePluginIntegration": "应用到Claude Code 插件",
+    "enableClaudePluginIntegration": "应用到 Claude Code 插件",
     "enableClaudePluginIntegrationDescription": "开启后可以在 Vscode Claude Code 插件里使用第三方供应商",
     "configFileLocation": "配置文件位置",
     "openFolder": "打开文件夹",