feat: Add service worker for asset caching

- Add Workbox-based service worker to cache JS, CSS, and fonts
- JS/CSS cached for 30 days, fonts for 1 year
- Support for CDN assets via ASSET_CDN_HOST
- Version-based cache invalidation on deployments
- Automatic cache cleanup for old versions
- Build service worker during asset precompilation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/javascript/dashboard/helper/pushHelper.js

@@ -16,7 +16,28 @@ export const verifyServiceWorkerExistence = (callback = () => {}) => {
 
   navigator.serviceWorker
     .register('/sw.js')
-    .then(registration => callback(registration))
+    .then(registration => {
+      // Check for updates on load
+      registration.update();
+
+      // Listen for updates
+      registration.addEventListener('updatefound', () => {
+        const newWorker = registration.installing;
+        newWorker.addEventListener('statechange', () => {
+          if (
+            newWorker.state === 'installed' &&
+            navigator.serviceWorker.controller
+          ) {
+            // New service worker available, will activate on next page load
+            console.log(
+              'New service worker available, will activate on next page load'
+            );
+          }
+        });
+      });
+
+      callback(registration);
+    })
     .catch(registrationError => {
       // eslint-disable-next-line
       console.log('SW registration failed: ', registrationError);