feat: First-class MCP server management UI (OAuth, test connection, toggle)

[rexlunae]

Inspiration: Unsloth

Unsloth's MCP integration is polished at the UI level:

• Built-in MCP servers: Context7, Exa, Hugging Face come pre-configured
• Custom MCP servers: Add via URL with OAuth sign-in or custom headers
• Test connection button: Validates the server is reachable before saving
• Per-server toggles: Enable/disable individual servers + a master toggle
• Auto tool discovery: Shows Refreshed "Vercel" (18 tools) on connect
• Edit/delete/refresh: Full CRUD on connected servers
• Multiple servers simultaneously: Use 3+ MCP servers in one chat

Current RustyClaw state

RustyClaw already has:

• MCP client (rmcp crate, feature flag)
• MCP manager (mcp/manager.rs)
• Config-based server definitions in rustyclaw.toml

But there's no UI for managing MCP servers — it's config-file only.

What RustyClaw could do

1. TUI MCP management: In the TUI, a "MCP Servers" panel showing connected servers, their tools, and status
2. Desktop MCP management: GUI panel with add/edit/delete/test flow
3. Dynamic add/remove: Add and remove MCP servers at runtime without editing config + restart
4. OAuth flow: For servers that require OAuth (GitHub MCP, etc.), open browser for authorization
5. Tool preview: Show what tools a server exposes before connecting
6. CLI: rustyclaw mcp add, rustyclaw mcp list, rustyclaw mcp test

Scope

• rustyclaw-tui: MCP management panel
• rustyclaw-desktop: MCP settings in desktop UI
• rustyclaw-core: Runtime MCP server add/remove/toggle (not just config at startup)
• rustyclaw-cli: rustyclaw mcp subcommand

Prior art

• Unsloth MCP guide: https://unsloth.ai/docs/basics/mcp.md
• Claude Desktop's MCP server config
• Continue.dev's MCP integration