fix: make checkAuthentication async to fix app.js parse error (#90)

checkAuthentication() used await for the OIDC exchange fetch but was
declared as a regular function, not async. This caused a JavaScript
syntax error that prevented the entire app.js file from parsing,
resulting in a completely non-interactive frontend after OIDC login.

Also bumped service worker cache version to v2 so browsers
discard stale cached JS files on the next load.

static/sw.js

@@ -1,5 +1,5 @@
 // OxiCloud Service Worker
-const CACHE_NAME = 'oxicloud-cache-v1';
+const CACHE_NAME = 'oxicloud-cache-v2';
 const ASSETS_TO_CACHE = [
   '/',
   '/index.html',