fix: Service Worker cache version v2 -> v3 to bust stale HTML cache

- SW cached / and /static/index.html with cache-first strategy
- Old HTML with 422 error persisted after rebuild
- Bumping CACHE_VERSION forces reinstall with fresh assets

frontend/sw.js

@@ -1,6 +1,6 @@
 /* ObsiGate Service Worker - PWA v2 with IndexedDB offline support */
 
-const CACHE_VERSION = 'obsigate-v2-pwa';
+const CACHE_VERSION = 'obsigate-v3-pwa';
 const STATIC_CACHE = CACHE_VERSION + '-static';
 const DYNAMIC_CACHE = CACHE_VERSION + '-dynamic';
 const MAX_DYNAMIC_CACHE_SIZE = 100;