feat: add "Last Online" column to client list and modal (Closes #3402) (#3405)

* feat: persist client last online and expose API

* feat(ui): show client last online in table and info modal

* i18n: add “Last Online” across locales

* chore: format timestamps as HH:mm:ss

web/translation/translate.en_US.toml

@@ -50,6 +50,7 @@
 "fail" = "Failed"
 "comment" = "Comment"
 "success" = "Successfully"
+"lastOnline" = "Last Online"
 "getVersion" = "Get Version"
 "install" = "Install"
 "clients" = "Clients"