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/users_admin_controller.go

@@ -378,7 +378,7 @@ func (c usersAdminController) FormAfterCreate(ctx context.Context, model any) er
 	vars["login"], vars["Login"] = email, email
 	hours := c.configInt("golem15.user.activation.activation_ttl_hours", 72)
 	vars["hours"], vars["Hours"] = hours, hours
-	template := mailName(ctx, user, "golem15.user::mail.invite")
+	template := mailName(ctx, app, user, "golem15.user::mail.invite")
 	lagoon.AfterCommit(ctx, tx, func(ctx context.Context, _ *gorm.DB) {
 		sendTemplateVars(ctx, app, template, email, vars)
 	})