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/defaults.go

@@ -16,6 +16,8 @@ package cookiebanner
 
 import "go.probo.inc/probo/pkg/coredata"
 
+var SupportedLanguages = []string{"en", "fr", "de", "es"}
+
 var defaultCategories = []struct {
 	Name        string
 	Slug        string