feat(auth): show OAuth-only registration notice on login page

Move the user creation check from OauthLoginService into
OauthSetting::allowsUserCreation(), so the login view can use it.
The login view now gets a can_register_with_oauth flag. The flag is
true when an enabled OAuth provider allows creating new users. The
view uses it to show the new auth.register_with_oauth message.
Add translations for the message in all locales, plus test coverage.

app/Models/OauthSetting.php

@@ -37,6 +37,19 @@ class OauthSetting extends Model
         );
     }
 
+    /**
+     * Only OIDC exposes a provider-level user creation setting. Other providers
+     * follow the instance registration setting, as before OIDC support.
+     */
+    public function allowsUserCreation(): bool
+    {
+        if (instanceSettings()->is_registration_enabled) {
+            return true;
+        }
+
+        return $this->provider === 'oidc' && $this->allow_registration;
+    }
+
     public function couldBeEnabled(): bool
     {
         switch ($this->provider) {