i18n: complete Korean (ko) translations

[moduvoice]

Motivation

This completes Korean (ko) translation coverage — to help Korean-speaking users use this
open-source project more comfortably in their own language. ko was already registered in
SUPPORTED_LOCALES and wired into the i18n loader, but messages/ko.json was missing a
large portion of keys (they were silently falling back to English via the deep-merge
fallback in src/i18n/request.ts).

한국 사람들의 오픈소스 이용에 도움이 되게 하기 위해서 한글화 작업을 완료하였습니다.

Changes

• messages/ko.json: translated all 277 keys that existed in messages/en.json but were
  missing from ko.json — primarily the Admin panel (Admin.*), the admin/self views of
  the user profile (User.Profile.admin.*, User.Profile.user.*, User.Profile.common.*),
  Agent.*, Auth.SignUp.*, Layout.*, MCP.*, and Workflow.* sections.
• Removed one stale key, Chat.Tool.toolModeDescription, that existed in the old
  ko.json but no longer exists in en.json (it has been superseded by the three
  separate autoToolModeDescription / manualToolModeDescription /
  noneToolModeDescription keys, which were already translated).
• No code changes — this is a translation-only, JSON-only change. ko was already present
  in SUPPORTED_LOCALES (src/lib/const.ts) and the dynamic-import + deep-merge fallback
  in src/i18n/request.ts needed no changes.

Testing

• Key parity: flattened both en.json and ko.json to their leaf keys and diffed
  them — confirmed 746/746 keys present, 0 missing, 0 extra.
• Placeholder integrity: extracted every {token} interpolation placeholder from each
  string and compared EN vs KO per key — all genuine variables ({name}, {role},
  {count}, {tokens}, {period}, etc.) are preserved. The only two "mismatches" a naive
  check flags come from the literal s inside the English ICU plural macro
  (model{count, plural, =1 {} other {s}}) in tokensAcross and totalCount — the Korean
  strings correctly collapse the plural macro into a single natural string since Korean
  nouns don't inflect for plural count. Verified this is valid by instantiating both
  translated strings with intl-messageformat (the library behind next-intl) at
  count=1 and count>1 — both parse and format correctly with no runtime errors.
• Formatting/lint/types: ran pnpm exec biome format messages/ko.json and
  pnpm exec biome lint messages/ko.json (both clean, no changes needed) and
  pnpm check-types (tsc --noEmit, passes with no errors).
• Manual spot-check: read through all 277 newly-translated keys against en.json,
  including the entire previously-missing Admin panel and User.Profile.admin/user
  subtrees, checking for mistranslation, register, and terminology consistency with the
  pre-existing 470 keys (e.g. 에이전트=Agent, 워크플로우=Workflow, MCP 서버, 역할=role,
  정지/정지 해제=ban/unban) — all consistent with established usage already present in the
  file before this change.

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