Add MoonProxy (frp desktop GUI client) to Networking Tools

[chenxxpro]

What is MoonProxy?

MoonProxy is a cross-platform desktop GUI client for frp (Fast Reverse Proxy). It lets non-technical users expose local services through their own frps server without touching config files or the command line.

Highlights:

• Visual proxy-rule management (TCP / UDP / HTTP / HTTPS)
• Real-time traffic monitoring (up/down speed, connection count, cumulative usage)
• One-click connect with 4-state status indicator (stopped / connecting / connected / error)
• System tray with background frpc operation
• Auto-launch, silent start, scheduled connect, and self-update of the frpc engine
• Built with Tauri v2 — runs on Windows and macOS

Where it's added

A new entry under Networking Tools & Concepts, following the existing [Name](url) is ... format used throughout that section. MoonProxy fits the guide's scope of core network components since it directly relates to NAT traversal, reverse proxying, and remote access — concepts already documented in this README (see the NAT and VPN bullet points in the same section).

Checklist

• Entry follows the existing [Name](url) is ... style
• Placed in the Networking Tools & Concepts section
• No other sections modified