Add admin panel, cart, checkout, and digital key delivery

Co-authored-by: Cursor <cursoragent@cursor.com>

app/templates/product.html

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+{% extends "base.html" %}
+{% block title %}{{ product.title }} — {{ app_name }}{% endblock %}
+
+{% block content %}
+<section class="page-section product-page">
+  <a class="back-link" href="/#catalog">← К каталогу</a>
+  <div class="product-layout">
+    <div class="product-hero-media gradient-{{ product.image_gradient }}">
+      <div class="product-media tall"></div>
+    </div>
+    <div class="product-info">
+      <p class="product-tag">{{ product.delivery_note }}</p>
+      <h1>{{ product.title }}</h1>
+      <p class="lead">{{ product.short_description }}</p>
+      <p>{{ product.description }}</p>
+      <p class="stock-line">В наличии: {{ stock }} шт.</p>
+      <div class="buy-row">
+        <span class="price big">{{ product.price | rub }}</span>
+        <form method="post" action="/cart/add/{{ product.id }}" class="inline-form">
+          <label class="qty-label">
+            Кол-во
+            <input type="number" name="quantity" min="1" max="{{ stock if stock > 0 else 1 }}" value="1" {% if stock < 1 %}disabled{% endif %} />
+          </label>
+          <button class="btn btn-primary" type="submit" {% if stock < 1 %}disabled{% endif %}>
+            {% if stock < 1 %}Нет в наличии{% else %}В корзину{% endif %}
+          </button>
+        </form>
+      </div>
+    </div>
+  </div>
+</section>
+{% endblock %}