Add a new translation locale

This skill adds a new locale to the Polar frontend i18n package (clients/packages/i18n) and the corresponding public docs. Each step produces its own commit so the PR reads cleanly.

Ask the user for the locale if they haven't provided one. You need:

• ${isoAlpha2} — the locale code used as the filename and config key (e.g. ko, ja, pt-PT). This is the value appended to SUPPORTED_LOCALES verbatim.
• ${localeName} — the human-readable name used in LOCALE_NAMES (e.g. Korean (South Korea), Japanese). Follow the style of the existing entries in clients/packages/i18n/src/config.ts.

Throughout this doc, ${isoAlpha2} is substituted literally into filenames, branch names, commit messages, and identifiers.

Step 1: Sync main

git checkout main
git pull --ff-only

Bail out if the working tree isn't clean — ask the user before stashing or discarding anything.