Always revalidate the app's files, tell open pages when a newer version exists, and log why a session is refused

An open page kept running the old version after a deploy (the single page never reloads), so the new
People screens did not appear. Static files are now sent with no-cache + ETag (304 when unchanged), icons
are cached for a week, /api/config reports a build stamp, and the app shows "A new version is available -
Reload" when it comes back into view or every 10 minutes. The service worker cache name is bumped.

The server also logs, at most once a minute per reason, why it refused a session cookie (unknown account,
turned off, signed out everywhere, bad signature, expired) without printing the cookie.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/sw.js

@@ -1,5 +1,5 @@
 // App-shell cache only; Seafile/API traffic always goes to the network.
-const CACHE = 'hub-v4';
+const CACHE = 'hub-v5';
 const SHELL = ['/style.css', '/app.js', '/icon.svg', '/manifest.webmanifest', '/icons/icon-192.png', '/icons/apple-touch-icon.png'];
 
 self.addEventListener('install', e => {