Add Korean (ko) locale

[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 config/locales/ko.yml (new file, 417 lines) with a full Korean translation of the admin UI, mirroring the structure of config/locales/en.yml exactly.
• No other files changed or need to change — Spina::Engine is a standard Rails::Engine with no custom i18n.load_path or locale allowlist, so config/locales/**/*.yml is auto-discovered the same way it already is for the repo's other 18 locale files (nl, de, fr, ru, zh-CN, etc.).
• Note: Spina.config.locales is a separate, unrelated concept — it configures which locales page content is translated into (Spina::TranslatedContent, page/resource translation UI), not which admin-UI languages are available. Admin UI language selection follows the host app's I18n.default_locale, same as every other existing locale file in this directory.
• The spina.languages map (used to render language names in the page-translation UI) was translated to Korean for all 16 existing entries, deliberately omitting a self-referential ko entry to match en.yml's own omission of en from... (actually en.yml does include en, matching exactly — key sets are identical, no additions or omissions beyond parity).

Testing

• Verified exact leaf-key parity between en.yml and ko.yml via a Ruby/YAML script (YAML.load_file + recursive key flattening): 359/359 leaf keys present in both, 0 missing, 0 extra.
• Verified every %{...} interpolation token matches exactly between corresponding en/ko keys: 0 mismatches.
• Verified HTML tag structure (tag name sequence) matches exactly for every *_html key: 0 mismatches.
• Verified spina.languages key set matches en.yml exactly (16 keys, same set, no ko self-entry in either file).
• Verified both YAML files parse successfully with Ruby's YAML.load_file (no syntax errors).
• Confirmed only one file is touched by this change (git diff --stat against main): config/locales/ko.yml, 417 insertions, no other files modified.
• Manually spot-checked 35+ translated keys across activerecord attributes, images, navigations, pages, page_translations, photos, preferences, resources, ui, user_mailer, users, website, and wysiwyg sections for accuracy, natural CMS-admin register, and correct Korean grammar (e.g. dynamic particle handling like %{subject}을(를)).