feat(dashboard): make sales chart period buttons work

- Replace placeholder setPeriod() with functional period switcher
- loadChart() supports Hoy (1 day), Semana (7 days), Mes (4 weeks), Año (12 months)
- Includes both normal POS sales and imported historical sales
- Updates chart title, total label and legend dynamically
- Bump dashboard.js to ?v=6

pos/templates/dashboard.html

@@ -568,7 +568,7 @@
   <script src="/pos/static/js/pos-utils.js?v=2" defer></script>
   <script src="/pos/static/js/sidebar.js" defer></script>
   <script src="/pos/static/js/dashboard-stats.js?v=3" defer></script>
-  <script src="/pos/static/js/dashboard.js?v=5" defer></script>
+  <script src="/pos/static/js/dashboard.js?v=6" defer></script>
   <script src="/pos/static/js/sync-engine.js" defer></script>
   <script>if('serviceWorker' in navigator){navigator.serviceWorker.register('/pos/sw.js',{scope:'/pos/'});}</script>
   <script src="/pos/static/js/pwa-install.js" defer></script>