fix(dashboard): bust PWA cache for historical sales section

- Bump service worker cache name to v18 to clear stale JS/CSS caches
- Bump dashboard.js to ?v=4 so browsers fetch updated script

pos/static/pwa/sw.js

@@ -6,7 +6,7 @@
 // The fetch handler normalizes static asset URLs (strips ?v= query strings)
 // so templates can use cache-busting query params freely.
 
-const CACHE_NAME = 'nexus-pos-v17';
+const CACHE_NAME = 'nexus-pos-v18';
 
 const APP_SHELL = [
   '/pos/static/css/tokens.css',