feat: implement user authentication features (registration, activation, and password reset)

- Added email templates for activation and password reset (`activation_email.html`, `password_reset_email.html`, etc.)
- Created forms for registration (`RegisterForm`) and login (`LoginForm`) with custom validation
- Developed new pages for account activation (`activation_success.html`, `activation_invalid.html`), password reset workflow, and profile
- Configured email backend and environment file handling in settings
- Updated header template to conditionally display profile or login links based on authentication status
- Enhanced CSS styles for authentication-related pages and forms

templates/users/emails/activation_email.html

@@ -0,0 +1,20 @@
+<html lang="ru">
+<body style="margin:0;padding:24px;font-family:Arial,sans-serif;background:#f5f7fa;color:#36213e;">
+<div style="max-width:640px;margin:0 auto;background:#ffffff;border:1px solid #d6dde6;padding:32px;">
+    <h1 style="margin:0 0 16px;font-size:28px;color:#36213e;">Подтверждение регистрации</h1>
+    <p style="margin:0 0 16px;font-size:16px;line-height:1.5;">
+        Здравствуйте{% if user.get_full_name %}, {{ user.get_full_name }}{% endif %}!
+    </p>
+    <p style="margin:0 0 24px;font-size:16px;line-height:1.5;">
+        Спасибо за регистрацию в {{ site_name }}. Подтвердите почту, чтобы активировать аккаунт.
+    </p>
+    <p style="margin:0 0 24px;">
+        <a href="{{ activation_url }}" style="display:inline-block;padding:14px 22px;background:#63768d;color:#ffffff;text-decoration:none;">Подтвердить почту</a>
+    </p>
+    <p style="margin:0;font-size:14px;line-height:1.5;color:#554971;">
+        Если кнопка не работает, скопируйте ссылку в браузер:<br>
+        <a href="{{ activation_url }}">{{ activation_url }}</a>
+    </p>
+</div>
+</body>
+</html>