refactor(mcp): redesign MCP management panel UI

- Redesign MCP panel to match main interface style
- Add toggle switch for each MCP server to enable/disable
- Use emerald theme color consistent with MCP button
- Create card-based layout with one MCP per row
- Add dedicated form modal for add/edit operations
- Implement proper empty state with friendly prompts
- Add comprehensive i18n support (zh/en)
- Extend McpServer type to support enabled field
- Backend already supports enabled field via serde_json::Value

Components:
- McpPanel: Main panel container with header and list
- McpListItem: Card-based list item with toggle and actions
- McpFormModal: Independent modal for add/edit forms
- McpToggle: Emerald-themed toggle switch component

All changes passed TypeScript type checking and production build.

src/components/mcp/McpToggle.tsx

@@ -0,0 +1,41 @@
+import React from "react";
+
+interface McpToggleProps {
+  enabled: boolean;
+  onChange: (enabled: boolean) => void;
+  disabled?: boolean;
+}
+
+/**
+ * Toggle 开关组件
+ * 启用时为淡绿色，禁用时为灰色
+ */
+const McpToggle: React.FC<McpToggleProps> = ({
+  enabled,
+  onChange,
+  disabled = false,
+}) => {
+  return (
+    <button
+      type="button"
+      role="switch"
+      aria-checked={enabled}
+      disabled={disabled}
+      onClick={() => onChange(!enabled)}
+      className={`
+        relative inline-flex h-6 w-11 items-center rounded-full transition-colors focus:outline-none focus:ring-2 focus:ring-emerald-500/20
+        ${enabled ? "bg-emerald-500 dark:bg-emerald-600" : "bg-gray-300 dark:bg-gray-600"}
+        ${disabled ? "opacity-50 cursor-not-allowed" : "cursor-pointer"}
+      `}
+    >
+      <span
+        className={`
+          inline-block h-4 w-4 transform rounded-full bg-white transition-transform
+          ${enabled ? "translate-x-6" : "translate-x-1"}
+        `}
+      />
+    </button>
+  );
+};
+
+export default McpToggle;