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/types.ts

@@ -60,6 +60,7 @@ export interface McpServer {
   args?: string[];
   env?: Record<string, string>;
   cwd?: string;
+  enabled?: boolean; // 是否启用该 MCP 服务器，默认 true
   [key: string]: any;
 }