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

controllers/registration.go

@@ -118,7 +118,7 @@ func FinishLogin(ctx context.Context, app *backpack.App, db *gorm.DB, user *mode
 			return err
 		}
 		user.DeletedAt = gorm.DeletedAt{}
-		sendTemplateVars(ctx, app, mailName(ctx, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
+		sendTemplateVars(ctx, app, mailName(ctx, app, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
 	}
 	if ip != "" {
 		if err := db.WithContext(ctx).Unscoped().Model(user).Update("last_ip_address", ip).Error; err != nil {