feat(account): show the account its own last sign-in

Settings → Account names the latest sign-in on any device (only the
admin's user list did, though the 0.88 notes said otherwise), and the role
in the reader's language instead of the raw `user`.

frontend/src/i18n/locales/en/settings.json

@@ -53,6 +53,8 @@
   },
   "account": {
     "title": "Account",
+    "lastLogin": "Last sign-in: {{time}}",
+    "lastLoginNone": "No sign-in recorded yet",
     "youtubeAccess": "YouTube access",
     "youtubeAccessIntro": "Sign-in only shares your name and email. YouTube access is granted separately, below — each is optional and revocable anytime in your Google Account.",
     "readTitle": "Read subscriptions (your Feed)",