Fix NameError: oauth_check not defined in render_login()

The render_login() function was referencing oauth_check without defining it,
causing a 500 Internal Server Error whenever the login page was accessed.

Added oauth_check initialization from oauth_bb.oauth_check before rendering
the login template, with fallback to empty dict when OAuth is not available.

Error message:
  NameError: name 'oauth_check' is not defined. Did you mean: 'health_check'?

This bug was present regardless of OAuth configuration and would affect any
user accessing the login page.

Fixes: Discord user report of 500 error on login page

Author: crocodilestick

cps/web.py

@@ -1952,6 +1952,9 @@ def render_login(username="", password=""):
     if url_for("web.logout") == next_url:
         next_url = url_for("web.index")
 
+    # Get OAuth check status
+    oauth_check = oauth_bb.oauth_check if feature_support['oauth'] else {}
+
     # Get generic OAuth login button text for display
     generic_login_button = None
     if feature_support['oauth']: