fix(sites): So , Implemented BoardGameGeek using username validation API

- Added BoardGameGeek back using the new API endpoint suggested by @ppfeister
- Uses https://api.geekdo.com/api/accounts/validate/username?username={} for detection
- errorMsg checks for '"isValid":true' to detect valid usernames
- This approach avoids the previous issues with:
  * HTML parsing returning false positives
  * User API returning JSON with '[]' substrings that caused detection problems
- Successfully tested with both valid (blue) and invalid usernames

Thanks @ppfeister for the API suggestion and @akh7177 for the initial guidance

sherlock_project/resources/data.json

@@ -291,6 +291,14 @@
     "urlMain": "https://www.bookcrossing.com/",
     "username_claimed": "blue"
   },
+  "BoardGameGeek": {
+    "errorMsg": "\"isValid\":true",
+    "errorType": "message",
+    "url": "https://boardgamegeek.com/user/{}",
+    "urlMain": "https://boardgamegeek.com/",
+    "urlProbe": "https://api.geekdo.com/api/accounts/validate/username?username={}",
+    "username_claimed": "blue"
+  },
   "BraveCommunity": {
     "errorType": "status_code",
     "url": "https://community.brave.com/u/{}/",