Adds messages/ko.json — full next-intl translation covering all 12
namespaces (LoginPage, Sidebar, Header, ModeToggle, Dashboard, Inbox,
Contacts, Pipelines, Broadcasts, Automations, Flows, Settings).
Scoped to the language file only, per maintainer guidance on PR #362:
no changes to the default locale, component text, or any other file.
Verified 1:1 key parity with messages/en.json (1426/1426 leaf keys)
and exact preservation of all interpolation variables and HTML-ish
tags across every message.
