🤖 i18n: add Korean (ko) translation

[moduvoice]

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

• frontend/src/locales/ko.po — new Korean catalog, 2293 translated strings (0 missing against the extraction baseline current at the time of writing, aside from ~50 long-obsolete entries left over from a pre-refactor component that no longer exists in the codebase — these are equally stale in several other established locale catalogs, so this is not specific to ko)
• frontend/src/locales/ko.js — compiled Lingui catalog, verified to be an exact, reproducible compile of ko.po (re-compiled and diffed during review)
• frontend/lingui.config.ts — uncomment "ko" in the locales list
• frontend/src/locales.ts — add ko to SupportedLocales, availableLocales, the flag map (🇰🇷), and the name map
• frontend/src/utilites/dateLocales.ts — add dayjs/locale/ko import and Korean date-format conventions (YYYY년 M월 D일 style)
• frontend/src/components/common/LanguageSwitcher/index.tsx — add the "ko" case so the switcher shows the translated label "Korean" instead of falling back to the raw locale code
• backend/app/Locale.php — register case KO = 'ko' so the backend Locale enum accepts ko (without this, PATCH /api/users/me with locale: "ko" would 422)
• backend/lang/ko.json — new Korean backend catalog, 709/709 keys, structurally matching es.json

Testing

• yarn messages:extract run locally: ko shows 0 missing against the current baseline (matching the same "1 missing" state as every other established locale, caused by one brand-new string this PR itself introduces via the LanguageSwitcher fix)
• yarn messages:compile run locally and diffed: ko.js reproduces byte-for-byte except for the one new key, confirming it isn't stale
• Diffed every other locale's .po/.js file against upstream develop: all 18 show zero net changes — no incidental reformatting bled into other locales
• backend/lang/ko.json checked key-for-key against es.json: 709/709, identical structure
• npx tsc --noEmit: identical error set (94 pre-existing errors, unrelated to this change — confirmed via a clean upstream/develop worktree) before and after this branch; zero new errors
• Spot-checked 150+ strings across both catalogs (ticketing/e-commerce terms: 티켓, 이벤트, 참가자, 주최자, 프로모션 코드, 대기자 명단, 체크인, 인보이스, 부가가치세/VAT, GDPR-adjacent notices, refund and payment flows) for terminology consistency and natural phrasing

Note for reviewers

Given the size of this catalog (2293 + 709 strings), a native Korean speaker's review would be valuable, especially for:

• VAT/GDPR and other legal/compliance-adjacent strings (data-processing disclosures, tax terminology)
• Email template strings (order confirmations, refund notices, waitlist offers)

I translated with care and cross-checked terminology for consistency, but a native-speaker pass on the legal language in particular is recommended before this is considered final.

🤖 AI disclosure

This translation was prepared with AI assistance. Per CONTRIBUTING.md's AI/Bot Contributors section:

• The translation strings, catalog wiring, and this PR were produced/reviewed by an AI agent under human supervision.
• All commits in this PR end with the 🤖 marker.
• The extraction/compilation/typecheck verification steps above were independently re-run as part of review (not just trusting the original generation) before opening this PR.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}