feat: add Korean (ko) translation

[moduvoice]

Korean (ko) translation PR body template

Motivation

This adds Korean (ko) translation support — to help Korean-speaking users use this
open-source project more comfortably in their own language.

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

Changes

• Added locales/ko.json (new file, 502/502 keys — full parity with locales/en.json, identical key order, no missing/extra keys)
• Followed the 4-step process documented in locales/README.md:
  1. Added ko.json to locales/
  2. Added 'ko' to languageList[] and '한국어' to languageNameList[] in supporter.js
  3. Added 'ko' to languageCodeList[] in main.js
  4. Added 'ko-KR' (NSIS/electron-builder culture code, matching the existing zh-CN/zh-TW convention) to nsis.installerLanguages[] in res/builder/nsis.js

Two other files reference zh-CN/zh-TW but were intentionally left untouched as they are outside the scope of the locales/README.md guide:

• update.json — per-release announcement text (would require translating historical release notes rather than the app UI)
• res/builder/output/nsis.js — a separate, unused build config (package.json's nsis script points at res/builder/nsis.js, not this file); it also predates and lacks the arm64 support already present in res/builder/nsis.js

Testing

• Validated locales/ko.json with both python -c "import json" and node -e "JSON.parse(...)" — no syntax errors
• Compared every key against locales/en.json: 502/502 keys present, identical key order, 0 missing/extra
• Verified \r line-break escapes and %-style placeholders are preserved correctly in all translated strings
• Ran node --check on all 3 edited JS files (main.js, supporter.js, res/builder/nsis.js) — all pass (no lint/test scripts exist in this repo)
• Spot-checked ~47 translated strings across the file (UI labels, notifications, dialogs, statistics, WebDAV sync, locker/lock-mode settings, tour guide) for natural phrasing and consistent formal register appropriate for a desktop timer/productivity app

[RoderickQiu]

Hi, I highly appreciate your effort! It looks nice and I will be adding that for the next version that would be released within this month.