fix: guard i18n against unregistered locale

[NemeZZiZZ]

Problem

i18n() indexes the locale registry without a guard:

function i18n(key: string, locale: string): string {
  return locales[locale][key] ?? key   // locales[locale] === undefined → TypeError
}

Only en-US and zh-CN are registered by default, and Store.setLocale accepts any string without validation. So init(el, { locale: 'zh-TW' }) — or any typo — throws TypeError: Cannot read properties of undefined at the first tooltip render (CandleTooltipView.ts). The exception escapes from a canvas animation-frame callback (which has try/finally but no catch), leaving the frame half-drawn and the chart effectively broken from init.

Verified by execution against the real source: i18n('indicator.tooltip.open', 'zh-TW') → TypeError.

Fix

Two lines of defense:

1. i18n() never throws: locales[locale]?.[key] ?? key — unknown locale degrades to the key itself, same as an unknown key already does.
2. Store.setLocale() warns and falls back to en-US when the locale is not registered, so the chart still renders readable strings instead of raw keys.

registerLocale was already defensive ({ ...locales[locale], ...ls }); this makes the read side match.