feat: registration_enabled admin setting — hides signup surfaces, gates the API

One boolean (Security > Public Registration) switches the whole public
signup lifecycle: front page + login links, the logged-out sidebar's
Create-an-account (via a package-level templates hook so shared page
templates keep their signatures), GET /register -> 302 /login, and an
early 403 on POST /api/auth/register. First-user exception preserved:
zero admins keeps every route and link reachable for bootstrapping —
same 'users exist' reasoning as the setup gate. Admin user creation is
unaffected by design.

Live E2E verified: flag off hides all surfaces and blocks the POST
(403), flag on restores them (201); the admin UI renders the row
automatically.

templates/toast_templ.go

@@ -37,7 +37,7 @@ func ErrorToast(message string) templ.Component {
 			var templ_7745c5c3_Var2 string
 			templ_7745c5c3_Var2, templ_7745c5c3_Err = templ.ResolveAttributeValue("showErrorToast('" + message + "')")
 			if templ_7745c5c3_Err != nil {
-				return templ.Error{Err: templ_7745c5c3_Err, FileName: `toast.templ`, Line: 5, Col: 71}
+				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/toast.templ`, Line: 5, Col: 71}
 			}
 			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var2)
 			if templ_7745c5c3_Err != nil {