feat: login page polish — remove welcome text, add spacer, resize corner brand

- login.html: removed 'Добро пожаловать' heading, added 3.5rem spacer below logo
- style.css: .login-corner-brand font-size set to 1.5rem

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

app/templates/login.html

@@ -13,8 +13,8 @@
   <main class="center-box login-page">
     <section class="login-shell">
       <img src="/static/logo.png" alt="MONT" class="brand-logo brand-logo-fullscreen" />
-      <h1 class="login-title">Добро пожаловать</h1>
-      {% if session_notice %}<div class="session-notice">{{ session_notice }}</div>{% endif %}
+      <div style="height:3.5rem"></div>
+{% if session_notice %}<div class="session-notice">{{ session_notice }}</div>{% endif %}
       {% if login_error %}<div class="auth-error">{{ login_error }}</div>{% endif %}
       <form method="post" action="/login" class="panel login-panel">
       <input type="hidden" name="csrf_token" value="{{ csrf_token }}" />