feat(pwa): add install prompt banner and register in all POS templates

- pwa-install.js: captures beforeinstallprompt, shows dismissible
  banner with 7-day localStorage cooldown, handles appinstalled
- Registered in 12 POS templates alongside existing service worker

pos/templates/invoicing.html

@@ -1057,6 +1057,7 @@
   <script src="/pos/static/js/invoicing.js" 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>
 
   <script src="/pos/static/js/chat.js" defer></script>
 </body>