fix(quick-261029-d9j): pick the mail language from Accept-Language, add mail.fallback_locale, Polish subjects

- preferred_locale and the request locale are normalised (comma lists,
  q-weights, region subtags, case) to pl or en by NormalizeMailLocale, so a
  raw Accept-Language such as en-GB,en;q=0.9 selects the -en templates
- new golem15.user.mail.fallback_locale (default pl) is used when neither
  names a supported language; ResolveMailLocaleFor applies it, and Boot
  rejects any value other than pl or en
- activate, reactivate and restore Polish templates get Polish subjects
- no template, route or payload changes; MailTemplate and
  ResolveMailLocale keep their signatures

plugin.go

@@ -76,6 +76,9 @@ func (p *Plugin) Boot(app *backpack.App) error {
 	if err != nil {
 		return err
 	}
+	if _, err := classes.MailFallbackLocale(app); err != nil {
+		return err
+	}
 	// Serve publishes the database after Boot, so the GORM hooks, the
 	// blacklist store and its sweep are installed on lagoon.Publish, or at
 	// once when the database is already published.