feat(dashboard): add Korean language support

Adds Korean (ko) as a supported dashboard locale alongside en, zh-CN, vi, ja.
Browser locales starting with `ko` (e.g. `ko-KR`) normalize to `ko` and the
Language switcher exposes "Korean" / "한국어" as a selectable option.

The full ~2670-key translation block was translated from the English source
with the formal-polite register (하십시오체) appropriate for an enterprise
dashboard. Technical names (CCS, Claude, CLIProxy, OAuth, provider names,
env vars, paths) are kept untranslated. All placeholders (e.g. {{name}},
{{count}}, {{provider}}) are preserved verbatim, matching the parity test.

Also backfills 23 keys missing from zh-CN, vi, and ja that were added to en
with the recent "Shared Resource Controls" feature (#1206 -> 2a422b3b):
accountsPage.resources*, accountsTable.{sharedResources,resourcesTitle,
resourcesShared,resourcesProfileLocal,badges.profileLocal},
commonToast.resourcesUpdated, and the full editAccountSharedResources
section. Without this backfill the existing i18n parity test was red for
all non-en locales on dev.

Closes #1241

ui/tests/unit/ui/i18n/language-switcher.test.tsx

@@ -96,6 +96,7 @@ describe('Dashboard i18n', () => {
   it.each([
     { locale: 'vi', label: 'Vietnamese', browserLocale: 'vi-VN' },
     { locale: 'ja', label: 'Japanese', browserLocale: 'ja-JP' },
+    { locale: 'ko', label: 'Korean', browserLocale: 'ko-KR' },
   ])(
     'supports $locale locale in switcher and persistence',
     async ({ locale, label, browserLocale }) => {