Fix PR review comments on cookie banner i18n

Address locale normalization for region-tagged values, guard
language detection for non-DOM runtimes, validate DefaultLanguage
on update, pass translated texts through the deactivation flow,
handle slug collisions in migration, add organizations FK, fix
consent migration from name-keyed to slug-keyed data, render all
template placeholders in previews, and wrap helper text for i18n.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/cookiebanner/service.go

@@ -177,6 +177,7 @@ func (r *UpdateCookieBannerRequest) Validate() error {
 	v.Check(r.PrivacyPolicyURL, "privacy_policy_url", validator.URL())
 	v.Check(r.ConsentExpiryDays, "consent_expiry_days", validator.Min(1))
 	v.Check(r.ConsentMode, "consent_mode", validator.OneOfSlice(coredata.CookieConsentModes()))
+	v.Check(r.DefaultLanguage, "default_language", validator.OneOfSlice(SupportedLanguages))
 
 	return v.Error()
 }