feat(simplify): first-time guidance welcome card

Show a welcome card on the dashboard when no sync runs exist yet.
Guides new users: 1. Check Settings → 2. Start Sync → 3. Map SKUs.
Card auto-hides after first successful sync.

Cache-bust: dashboard.js?v=31, style.css?v=24

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/templates/dashboard.html

@@ -6,6 +6,8 @@
 {% block content %}
 <h4 class="mb-4">Panou de Comanda</h4>
 
+<div id="welcomeCard" style="display:none"></div>
+
 <!-- Sync Card (unified two-row panel) -->
 <div class="sync-card">
   <!-- TOP ROW: Status + Controls -->
@@ -121,5 +123,5 @@
 {% endblock %}
 
 {% block scripts %}
-<script src="{{ request.scope.get('root_path', '') }}/static/js/dashboard.js?v=30"></script>
+<script src="{{ request.scope.get('root_path', '') }}/static/js/dashboard.js?v=31"></script>
 {% endblock %}