Connect guide: new canonical MCP URL, Claude Code CLI, and mcp-remote instructions

The MCP connect guide on Help and Settings now shows the dedicated proxy URL (vello-mcp.matteo-3bc.workers.dev) as the single canonical server address — replacing the internal Supabase function URL that spec-strict clients like mcp-remote couldn't use for OAuth. Two new connection sections have been added: step-by-step instructions for Claude Code CLI (claude mcp add --transport http) and a ready-to-paste mcp-remote config snippet for clients that only support local stdio servers.