Fix GrampsWeb login: use JSON body for /api/token/ endpoint (STI-104)

GrampsWeb expects JSON for the token endpoint, not form-encoded data.
Discovered during live API testing against production.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

app/gramps_mcp_server/client.py

@@ -45,7 +45,7 @@ class GrampsWebClient:
     def _login(self) -> None:
         """Authentifizierung bei GrampsWeb und Token-Speicherung."""
         login_endpoints = [
-            ("/api/token/", "form"),
+            ("/api/token/", "json"),
             ("/api/login/", "json"),
         ]
         for path, mode in login_endpoints: