Offline access: read recent mail without a connection

An opt-in per device (Settings → Security → Offline access) installs a
service worker that keeps the newest 50 Inbox conversations, and the ones
opened recently, for reading when the server cannot be reached. While
online everything still comes from the server; the saved copy is only the
fallback, marked X-WM-Offline so the app knows it is offline.

Offline, an amber cloud next to the theme toggle says so and when mail was
last saved; actions that need the server are greyed out and say "Not
available offline"; conversations and folders not saved here are faded or
say so. A probe notices the server coming back, shows "Back online" and
refreshes everything. Saved mail is wiped on sign-out, on any 401, and when
another user signs in. nginx never caches /sw.js, so releases reach it.

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

docker/nginx-app.conf

@@ -76,3 +76,10 @@ location = /healthz {
 location = /index.html {
     add_header Cache-Control "no-store, must-revalidate";
 }
+
+# The offline-access service worker (frontend/public/sw.js): a fixed URL the
+# browser re-checks for a new version, so it must not be cached either.
+location = /sw.js {
+    add_header Cache-Control "no-cache";
+    try_files $uri =404;
+}