Convert to Flask app with password-protected admin panel

Links now live in data/links.json instead of hardcoded HTML, editable
via /admin (add/rename/reorder/delete sections and links, per-link
color and half-width side-by-side layout). New pages can be added
without code changes. Docker image only needs rebuilding on code
changes now, not on content changes (data/ is volume-mounted).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/admin_login.html

@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<html lang="de">
+<head>
+  <meta charset="UTF-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
+  <title>Admin Login</title>
+  <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
+  <style>
+    .admin-box {
+      max-width: 320px;
+      margin: 80px auto;
+      padding: 24px;
+      background: rgba(255, 255, 255, 0.04);
+      border: 1px solid #4b5563;
+      border-radius: 10px;
+    }
+    .admin-box label { display: block; margin-bottom: 6px; color: #9ca3af; font-size: 13px; }
+    .admin-box input {
+      width: 100%;
+      padding: 10px;
+      margin-bottom: 16px;
+      border-radius: 6px;
+      border: 1px solid #4b5563;
+      background: #0f172a;
+      color: #f3f4f6;
+    }
+    .admin-box button {
+      width: 100%;
+      padding: 10px;
+      border-radius: 6px;
+      border: none;
+      background: #21e4d3;
+      color: #0f172a;
+      font-weight: 600;
+      cursor: pointer;
+    }
+    .admin-box button:hover { background: #24ffed; }
+    .admin-error { color: #fca5a5; margin-bottom: 12px; font-size: 14px; }
+  </style>
+</head>
+<body>
+  <div class="admin-box">
+    <h1 style="font-size: 20px; margin-bottom: 16px; text-align: left;">Admin Login</h1>
+    {% if error %}<div class="admin-error">{{ error }}</div>{% endif %}
+    <form method="post" action="{{ url_for('admin_login') }}">
+      <label for="password">Passwort</label>
+      <input type="password" id="password" name="password" autofocus required>
+      <button type="submit">Einloggen</button>
+    </form>
+  </div>
+</body>
+</html>