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

config.example.json

@@ -36,6 +36,9 @@
   "maxAccounts": 10,
   "_maxAccounts_comment": "Maximum number of Google accounts allowed (1-100). Default: 10.",
 
+  "_oauthCallbackPort_comment": "OAuth callback server port. Change if you get EACCES errors on Windows. Can also use OAUTH_CALLBACK_PORT env var. Default: 51121.",
+  "_oauthCallbackPort_env": "OAUTH_CALLBACK_PORT=3456",
+
   "_profiles": {
     "development": {
       "debug": true,