Add user authentication and registration features

- Implement login and registration HTML templates for user authentication.
- Create user management functions in user.py for handling user data, including adding, deleting, and updating users.
- Introduce password hashing and strength checking for secure user credentials.
- Add terms of service and privacy policy pages to the website.
- Enhance main page with dynamic content based on user session status.

Website/templates/register.html

@@ -0,0 +1,103 @@
+{% extends "base.html" %}
+
+{% block title %}Registrieren{% endblock %}
+
+{% block head %}
+{{ super() }}
+<style>
+    .auth-wrap {
+        max-width: 560px;
+        margin: 2rem auto;
+        padding: 2rem;
+        border-radius: 18px;
+        border: 1px solid #d4e0e9;
+        background: #ffffff;
+        box-shadow: 0 20px 44px rgba(14, 32, 48, 0.08);
+    }
+
+    .auth-wrap h1 {
+        font-size: 2rem;
+        color: #0f344d;
+        margin-bottom: 0.4rem;
+    }
+
+    .auth-wrap p {
+        margin-bottom: 1.3rem;
+    }
+
+    .field {
+        margin-bottom: 0.9rem;
+    }
+
+    .field label {
+        display: block;
+        margin-bottom: 0.35rem;
+        font-weight: 700;
+        color: #19445f;
+    }
+
+    .field input {
+        width: 100%;
+        padding: 0.68rem 0.78rem;
+        border-radius: 10px;
+        border: 1px solid #c4d5e2;
+        font-size: 1rem;
+        color: #12384f;
+    }
+
+    .field input:focus {
+        outline: none;
+        border-color: #2f79a7;
+        box-shadow: 0 0 0 3px rgba(58, 133, 180, 0.2);
+    }
+
+    .auth-btn {
+        width: 100%;
+        border: none;
+        border-radius: 999px;
+        padding: 0.72rem;
+        font-size: 1rem;
+        font-weight: 700;
+        color: #ffffff;
+        background: linear-gradient(120deg, #0a5f8f 0%, #0b4567 100%);
+        cursor: pointer;
+    }
+
+    .auth-meta {
+        margin-top: 1rem;
+        color: #547184;
+    }
+
+    .auth-meta a {
+        color: #0a5f8f;
+        font-weight: 700;
+    }
+</style>
+{% endblock %}
+
+{% block content %}
+<section class="auth-wrap">
+    <h1>Registrierung</h1>
+    <p>Legen Sie ein Konto an, um den Kalender fuer Terminanfragen zu nutzen.</p>
+    <form method="POST" action="{{ url_for('register') }}">
+        <div class="field">
+            <label for="display_name">Name</label>
+            <input id="display_name" name="display_name" type="text" autocomplete="name" required>
+        </div>
+        <div class="field">
+            <label for="username">Benutzername</label>
+            <input id="username" name="username" type="text" autocomplete="username" required>
+        </div>
+        <div class="field">
+            <label for="password">Passwort (mind. 8 Zeichen)</label>
+            <input id="password" name="password" type="password" autocomplete="new-password" required>
+        </div>
+        <div class="field">
+            <label for="password_repeat">Passwort wiederholen</label>
+            <input id="password_repeat" name="password_repeat" type="password" autocomplete="new-password" required>
+        </div>
+        <button class="auth-btn" type="submit">Konto erstellen</button>
+    </form>
+    <p class="auth-meta">Bereits registriert? <a href="{{ url_for('login') }}">Zum Login</a></p>
+</section>
+{% endblock %}