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

src/constants.js

@@ -171,13 +171,19 @@ export function isThinkingModel(modelName) {
 }
 
 // Google OAuth configuration (from opencode-antigravity-auth)
+// OAuth callback port - configurable via environment variable for Windows compatibility (issue #176)
+// Windows may reserve ports in range 49152-65535 for Hyper-V/WSL2/Docker, causing EACCES errors
+const OAUTH_CALLBACK_PORT = parseInt(process.env.OAUTH_CALLBACK_PORT || '51121', 10);
+const OAUTH_CALLBACK_FALLBACK_PORTS = [51122, 51123, 51124, 51125, 51126];
+
 export const OAUTH_CONFIG = {
     clientId: '1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com',
     clientSecret: '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',
     authUrl: 'https://accounts.google.com/o/oauth2/v2/auth',
     tokenUrl: 'https://oauth2.googleapis.com/token',
     userInfoUrl: 'https://www.googleapis.com/oauth2/v1/userinfo',
-    callbackPort: 51121,
+    callbackPort: OAUTH_CALLBACK_PORT,
+    callbackFallbackPorts: OAUTH_CALLBACK_FALLBACK_PORTS,
     scopes: [
         'https://www.googleapis.com/auth/cloud-platform',
         'https://www.googleapis.com/auth/userinfo.email',