feat: Gluetun VPN integration and remote service enhancements

Major features:
- Native Docker-based Gluetun VPN proxy provider with multi-provider support
  (NordVPN, Windscribe, Surfshark, ExpressVPN, and 50+ more)
- Stateless remote service architecture with local session caching
- Client-side authentication for remote services (browser, 2FA, OAuth support)

Key changes:
- core/proxies/windscribevpn.py: Enhanced proxy handling
- core/crypto.py: Cryptographic utilities
- docs/VPN_PROXY_SETUP.md: Comprehensive VPN/proxy documentation

unshackle/commands/dl.py

@@ -672,6 +672,8 @@ class dl:
                     self.log.info(f"Loaded {proxy_provider.__class__.__name__}: {proxy_provider}")
 
             if proxy:
+                # Store original proxy query for service-specific proxy_map
+                original_proxy_query = proxy
                 requested_provider = None
                 if re.match(r"^[a-z]+:.+$", proxy, re.IGNORECASE):
                     # requesting proxy from a specific proxy provider