Localize server-generated email notifications and confirmation emails

[Copilot]

Email subjects and bodies were still hard-coded in English, so recipients always received English emails regardless of the plugin locale setup. This change moves email content into the existing locale files and carries the subscriber’s language through the subscription flow so later notifications use the same locale.

• Email i18n

  • Added locale-backed strings for:
    • subscription confirmation emails
    • unsubscription confirmation emails
    • edit-start notifications
    • edit-end notifications
    • shared email footer text
  • Reused the existing locales/*.json structure instead of introducing a separate translation path.

• Server-side template resolution

  • Added emailTemplates.js to resolve locale, apply fallback to en, and interpolate dynamic values such as padId, padUrl, and confirmation links.
  • Updated handleMessage.js and update.js to build email subject/body from localized templates rather than inline strings.

• Persist recipient locale

  • Captured the client language during subscribe/unsubscribe requests.
  • Stored the resolved locale alongside subscription metadata so follow-up edit notifications are sent in the recipient’s selected language.

• Fallback behavior

  • Normalized locale values such as fr-CA / de-DE to supported plugin locales.
  • Defaulted unsupported locales to English.

const localizedEmail = getConfirmationEmail({
  action: 'subscribe',
  locale: getUserLocale(userInfo),
  padId,
  padUrl: padUrl(padId),
  token: subscribeId,
});

await server.send({
  text: localizedEmail.text,
  subject: localizedEmail.subject,
  from: `${fromName} <${fromEmail}>`,
  to: userInfo.email,
});