feat: VivesPOS landing on Winter CMS 1.2 — theme + plugin + Dockerfile

- Base: wintercms/winter branch 1.2 (full framework)
- Theme vivespos: Canvas 7 + Bootstrap 5 CDN, custom CSS
- Layout: deferred GTM/GA4 tracking, JSON-LD SoftwareApplication
- Partials: hero (offline-first), features, modes (offline/nube toggle),
  screenshots, pricing (3 planes), comparison, FAQ, CTA
- Plugin VivesPOS.Site with ContactForm
- Dockerfile: PHP 8.2 Apache, port 80, healthcheck
- Added winter/wn-pages, blog, sitemap, seo plugins
- Active theme set to vivespos

themes/demo/partials/site/header.htm

@@ -0,0 +1,29 @@
+==
+function onStart()
+{
+    $this['backendUrl'] = Backend::url('/');
+}
+==
+<!-- Nav -->
+<nav id="layout-nav" class="navbar navbar-inverse navbar-fixed-top navbar-autohide" role="navigation">
+    <div class="container">
+        <div class="navbar-header">
+            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-main-collapse">
+                <span class="sr-only">Toggle navigation</span>
+                <span class="icon-bar"></span>
+                <span class="icon-bar"></span>
+                <span class="icon-bar"></span>
+            </button>
+            <a class="navbar-brand" href="{{ 'home'|page }}">Winter Demo</a>
+        </div>
+        <div class="collapse navbar-collapse navbar-main-collapse">
+            <ul class="nav navbar-nav">
+                <li class="separator hidden-xs"></li>
+                <li class="{% if this.page.id == 'home' %}active{% endif %}"><a href="{{ 'home'|page }}">Basic concepts</a></li>
+                <li class="{% if this.page.id == 'ajax' %}active{% endif %}"><a href="{{ 'ajax'|page }}">AJAX framework</a></li>
+                <li class="{% if this.page.id == 'plugins' %}active{% endif %}"><a href="{{ 'plugins'|page }}">Plugin components</a></li>
+                <li><a href="{{ backendUrl }}">Sign in to the Backend</a></li>
+            </ul>
+        </div>
+    </div>
+</nav>