feat(i18n): add Korean (ko) locale

[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

• Add web-common/src/lib/i18n/messages/ko.json with a full Korean message catalog (1786 keys), matching en.json key-for-key, including pluralized/variant messages.
• Register ko in web-common/src/lib/i18n/project.inlang/settings.json (locales: ["en", "es", "ko"]).
• Add a ko entry (한국어) to LanguageSwitcher.svelte.
• Add the language_ko key to en.json and es.json so the switcher can render "한국어" as a label regardless of the active locale (mirrors how language_es was added alongside en.json when Spanish was introduced in feat(i18n): add Spanish locale with full EN/ES translations #9635).

Testing

• node scripts/i18n-guard.js --strict — catalog integrity (key parity across locales, no empty values, placeholder/variant consistency) passes.
• npm run build:i18n (paraglide-js compile) — compiles cleanly and generates correct per-locale message functions for ko.
• npm run test -w web-common -- --run src/components/i18n — LanguageSwitcher.spec.ts passes.
• npm run quality (web-common scope) — no new lint/type errors introduced by this change (2 pre-existing, unrelated TS errors in ExportDashboardForm.svelte are present on main before this PR and untouched by it).
• Scripted diff of every message: verified each value's {placeholder} set, and every pluralized message's declarations/selectors/match shape, are identical to en.json — only the Korean text differs.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code