Offline access: self-updating worker, quiet console, setting in Accounts

- The page asks the service worker for its version before using it, and
  has the browser fetch a newer one when it is out of date; the app also
  checks for a new sw.js on every start. An older worker left over from a
  previous release silently ignored the page's requests, so saving only
  resumed after offline access was switched off and on.
- Fewer console errors offline: the server probe goes through the worker;
  fonts, the illustration, lazy chunks and the PDF worker (named inside
  the app's scripts and styles) are saved with the app; sender pictures
  are pre-loaded, and an unsaved one is "no picture" (204), not a 503;
  plugin bundles and plugin lookups are saved (per-message ones go with
  their message); POST lookups and live-update reconnects wait for the
  server to be back instead of failing over and over.
- The setting moves from Settings → Security to Settings → Accounts.

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

frontend/src/i18n/en.ts

@@ -550,7 +550,7 @@ export const en = {
   "offline.explainSaved": "Offline — showing mail saved on this device, last updated {when}.",
   "offline.explainSavedNoTime": "Offline — showing mail saved on this device.",
   "offline.explainNotSaved":
-    "Can't reach the server. To read recent mail without a connection, turn on offline access in Settings → Security.",
+    "Can't reach the server. To read recent mail without a connection, turn on offline access in Settings → Accounts.",
   "offline.noticeSaved": "You're offline. Recent mail is still available.",
   "offline.noticeNotSaved": "You're offline — can't reach the server.",
   "offline.notAvailable": "Not available offline",