Add default-server ntfy auth (fix 403 on protected topics)

The "send test" action, admin alerts and feeds without their own
credentials now use configurable default-server token / basic auth,
so publishing works against ntfy servers with access control enabled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/static/app.js

@@ -410,6 +410,9 @@ sForm.addEventListener("submit", async e => {
   e.preventDefault();
   const payload = {
     default_ntfy_server: sForm.default_ntfy_server.value.trim(),
+    default_ntfy_token: sForm.default_ntfy_token.value.trim(),
+    default_ntfy_username: sForm.default_ntfy_username.value.trim(),
+    default_ntfy_password: sForm.default_ntfy_password.value,
     check_interval: parseInt(sForm.check_interval.value, 10),
     auth_enabled: sForm.auth_enabled.checked,
     telegram_enabled: sForm.telegram_enabled.checked,