fix: enable dynamic MCP server start/stop without app restart

[52sanmao]

Summary

• mcp-server:saveConfig handler now dynamically starts/stops the MCP server after saving config, instead of requiring a full app restart
• Follows the same pattern already used by the MITM proxy saveConfig handler
• Updated UI toast message to remove "重启应用后生效" since changes now take effect immediately

Problem

When users enable the MCP Server toggle and click "保存 MCP Server 设置", the config is saved to disk but the server doesn't actually start. The UI shows "已停止" even though enabled: true. Users must restart the entire app for the server to start. (See #54)

Root Cause

The mcp-server:saveConfig IPC handler only called saveMCPServerConfig(config) without invoking initMCPServer(). The MCP server was only initialized once at app startup (app.whenReady).

Fix

Added dynamic start/stop logic to saveConfig handler (matching the existing MITM proxy pattern):

if (config.enabled && !isMCPServerRunning()) {
  await initMCPServer(...);
} else if (!config.enabled && isMCPServerRunning()) {
  await stopMCPServer();
}

Closes #54