Logo in system mail; "don't ask for the 2FA code on this device"

System mail: the password-reset email (and the admin panel's test email) now
opens with the Ultimo logo, centered. It is a PNG rendered from
icons/ultimo-logo.svg and embedded in the message (multipart/related, cid:),
so it shows where remote images are blocked and in clients that do not
render SVG. The PNG is listed in package-data so the Docker image carries it.

Trusted devices: a 2FA sign-in offers "Don't ask for the code on this device
in future". Ticked with a correct code, the browser gets its own HttpOnly
trusted-device cookie (token in Redis, 90 days, renewed on use); later
sign-ins to the same account from that browser need only the password. Other
accounts and browsers are unaffected. The trust is revoked on a password
reset, when 2FA is turned off, and when an admin blocks or deletes the user;
signing out keeps it.

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

frontend/src/i18n/en.ts

@@ -29,6 +29,7 @@ export const en = {
   "auth.passwordHint": "at least 8 characters",
   "auth.totpCode": "2FA code",
   "auth.totpPrompt": "Enter the 6-digit code from your authenticator app",
+  "auth.rememberDevice": "Don't ask for the code on this device in future",
   "auth.signIn": "Sign in",
   "auth.signingIn": "Signing in…",
   "auth.create": "Create account",