UI polish: global modern styles; Admin button pinned top-left across app and generated pages; index redirects to login for guests

templates/login.html

@@ -0,0 +1,16 @@
+{% extends 'base.html' %}
+{% block content %}
+  <h1>Вход в админку</h1>
+  <form method="post">
+    <div class="row">
+      <label>Имя пользователя</label>
+      <input type="text" name="username" autocomplete="username" required>
+    </div>
+    <div class="row">
+      <label>Пароль</label>
+      <input type="password" name="password" autocomplete="current-password" required>
+    </div>
+    <button class="btn" type="submit">Войти</button>
+  </form>
+{% endblock %}
+