Fix brand name displaying as uppercase on auth pages

Remove text-transform: uppercase from .brand CSS in success and error
HTML templates so "mfbt" renders in lowercase as intended.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: shuveb

src/mfbt/auth.py

@@ -161,7 +161,6 @@ class CallbackResult:
     font-size: 14px;
     font-weight: 600;
     letter-spacing: 0.08em;
-    text-transform: uppercase;
     color: #58a6ff;
     margin-bottom: 28px;
   }
@@ -230,7 +229,6 @@ class CallbackResult:
     font-size: 14px;
     font-weight: 600;
     letter-spacing: 0.08em;
-    text-transform: uppercase;
     color: #58a6ff;
     margin-bottom: 28px;
   }}