fix: make OAuth callback port configurable for Windows compatibility (#176)

- Add OAUTH_CALLBACK_PORT environment variable (default: 51121)
- Implement automatic port fallback (51122-51126) on EACCES/EADDRINUSE
- Add Windows-specific troubleshooting in error messages and README
- Document configuration in config.example.json

Closes #176

README.md

@@ -427,6 +427,64 @@ npm run test:strategies    # Account selection strategies
 
 ## Troubleshooting
 
+### Windows: OAuth Port Error (EACCES)
+
+On Windows, the default OAuth callback port (51121) may be reserved by Hyper-V, WSL2, or Docker. If you see:
+
+```
+Error: listen EACCES: permission denied 0.0.0.0:51121
+```
+
+The proxy will automatically try fallback ports (51122-51126). If all ports fail, try these solutions:
+
+#### Option 1: Use a Custom Port (Recommended)
+
+Set a custom port outside the reserved range:
+
+```bash
+# Windows PowerShell
+$env:OAUTH_CALLBACK_PORT = "3456"
+antigravity-claude-proxy start
+
+# Windows CMD
+set OAUTH_CALLBACK_PORT=3456
+antigravity-claude-proxy start
+
+# Or add to your .env file
+OAUTH_CALLBACK_PORT=3456
+```
+
+#### Option 2: Reset Windows NAT
+
+Run as Administrator:
+
+```powershell
+net stop winnat
+net start winnat
+```
+
+#### Option 3: Check Reserved Ports
+
+See which ports are reserved:
+
+```powershell
+netsh interface ipv4 show excludedportrange protocol=tcp
+```
+
+If 51121 is in a reserved range, use Option 1 with a port outside those ranges.
+
+#### Option 4: Permanently Exclude Port (Admin)
+
+Reserve the port before Hyper-V claims it (run as Administrator):
+
+```powershell
+netsh int ipv4 add excludedportrange protocol=tcp startport=51121 numberofports=1
+```
+
+> **Note:** The server automatically tries fallback ports (51122-51126) if the primary port fails.
+
+---
+
 ### "Could not extract token from Antigravity"
 
 If using single-account mode with Antigravity: