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/password_reset_complete.html

@@ -0,0 +1,33 @@
+{% extends '_layouts/base.html' %}
+{% load static %}
+
+{% block title %}Пароль обновлён{% endblock %}
+
+{% block add_css %}
+    <link rel="stylesheet" href="{% static 'css/login.css' %}">
+{% endblock %}
+
+{% block content %}
+    <div class="auth-page">
+        {% include "_fragments/header.html" %}
+        <main class="auth-section">
+            <div class="container auth-layout">
+                <section class="auth-intro auth-intro--login">
+                    <p class="auth-kicker">Готово</p>
+                    <h1 class="auth-title">Пароль обновлён</h1>
+                    <p class="auth-description">
+                        Новый пароль сохранён. Теперь можно войти в аккаунт с обновлёнными данными.
+                    </p>
+                </section>
+                <section class="auth-panel">
+                    <div class="auth-copy-block">
+                        <h2 class="auth-panel-title">Вход в аккаунт</h2>
+                        <p class="auth-panel-text">Используйте новый пароль на странице входа.</p>
+                        <a class="auth-submit-link" href="{% url 'users:login' %}">Перейти ко входу</a>
+                    </div>
+                </section>
+            </div>
+        </main>
+        {% include "_fragments/footer.html" %}
+    </div>
+{% endblock %}