UI: Simplify login page styling, remove purple gradient background, stop global .container overrides; add scoped .auth-wrapper; neutralize buttons/cards; rebuild verified via smoke test

app/templates/login.html

@@ -3,7 +3,7 @@
 {% block title %}Login - Delphi Database{% endblock %}
 
 {% block content %}
-<div class="container">
+<div class="container auth-wrapper">
     <div class="row justify-content-center">
         <div class="col-md-8 col-lg-5">
             <div class="card shadow-lg" style="border: none; border-radius: 15px;">