[FEATURE]: Add MCP server setup and connection testing to Desktop

Feature hasn't been suggested before.

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Describe the enhancement you want to request

I use MCP servers with OpenCode Desktop, but adding or editing them still requires using the CLI or manually changing the configuration file.

It would be helpful to have a simple MCP settings page in the Desktop app where users can add, edit, remove, enable, and disable local or remote MCP servers.

The main feature I need is a Test Connection button. When a connection fails, the UI should show whether the command or endpoint could be reached, whether authentication succeeded, which tools were discovered, and the actual error message.

The Desktop UI should continue using the same MCP configuration as the CLI, so changes made from either interface remain consistent.

This would make the existing MCP support much easier to configure and troubleshoot, especially for users who mainly use the Desktop app.