Fix UnboundLocalError in render_login() caused by redundant oauth_bb import

The function was attempting to import oauth_bb locally inside the function body,
which caused Python to treat oauth_bb as a local variable throughout the entire
function scope. This resulted in an UnboundLocalError when trying to access
oauth_bb.oauth_check on line 1956, before the import statement on line 1965.

The oauth_bb module is already imported at the module level (lines 69-90) with
proper fallback handling via MockOAuth when OAuth is unavailable. The local
import inside the function is redundant and breaks name resolution.

Solution: Remove the redundant `from . import oauth_bb` import from inside the
render_login() function. The module-level import is sufficient for all uses
within the function.

Fixes: 500 Internal Server Error on login page
Caused by: Commits 29de7bd (import refactoring) and 7c49f5c (incomplete fix)

Related issues:
- Commit 29de7bd: "Fix language selection missing when OAuth/OIDC enabled"
- Commit 7c49f5c: "Fix NameError: oauth_check not defined in render_login()"

Author: crocodilestick

CONTRIBUTORS

@@ -312,7 +312,7 @@ Copyright (C) 2024-2026 Calibre-Web Automated contributors
 - zhiyue (1 commits)
 # Fork Contributors (crocodilestick/calibre-web-automated)
 
-- crocodilestick (896 commits)
+- crocodilestick (899 commits)
 - jmarmstrong1207 (73 commits)
 - demitrix (30 commits)
 - sirwolfgang (29 commits)
@@ -383,6 +383,7 @@ Copyright (C) 2024-2026 Calibre-Web Automated contributors
 - marauder37 (1 commits)
 - Marodeur80 (anon) (1 commits)
 - Matteo Benaroyo (anon) (1 commits)
+- mehalter (1 commits)
 - morpheus65535 (1 commits)
 - n00b42 (1 commits)
 - Nementon (1 commits)

cps/web.py

@@ -1960,7 +1960,6 @@ def render_login(username="", password=""):
     if feature_support['oauth']:
         try:
             # oauth_bb is already imported at module level, access oauthblueprints from it
-            from . import oauth_bb
             # oauthblueprints[2] is the generic OIDC provider (index 0=github, 1=google, 2=generic)
             if hasattr(oauth_bb, 'oauthblueprints') and len(oauth_bb.oauthblueprints) > 2:
                 generic_login_button = oauth_bb.oauthblueprints[2].get('login_button') or 'OpenID Connect'