fix: bust service worker cache on each build with version+timestamp

The SW cache name was hardcoded to 'pinchchat-v1', meaning PWA users
would get stale cached assets forever. Now the cache name includes
the package version and build timestamp, so each build creates a new
cache and old ones are cleaned up on activation.

Also adds:
- Periodic SW update check (every 30 min)
- Auto-reload when new SW version is detected

src/main.tsx

@@ -8,7 +8,24 @@ import './index.css'
 // Register service worker for PWA support (offline caching, installability)
 if ('serviceWorker' in navigator) {
   window.addEventListener('load', () => {
-    navigator.serviceWorker.register('/sw.js').catch(() => {
+    navigator.serviceWorker.register('/sw.js').then((reg) => {
+      // Check for updates periodically (every 30 min)
+      setInterval(() => reg.update().catch(() => {}), 30 * 60 * 1000);
+
+      // When a new SW is installed and waiting, reload to activate it
+      reg.addEventListener('updatefound', () => {
+        const newWorker = reg.installing;
+        if (!newWorker) return;
+        newWorker.addEventListener('statechange', () => {
+          if (newWorker.state === 'installed' && navigator.serviceWorker.controller) {
+            // New version available — reload on next navigation
+            // The new SW called skipWaiting(), so it activates immediately
+            // Reload to pick up cached assets from the new cache version
+            window.location.reload();
+          }
+        });
+      });
+    }).catch(() => {
       // SW registration failed — app works fine without it
     });
   });