refactor(settings): add registry-backed navigation and finder (#6040)

* refactor(settings): add modular shell primitives

* refactor(settings): wire modular shell

* test(settings): exercise real coordinator ESM boundary

* refactor(settings): add registry-backed settings finder

* fix(settings): harden registry navigation behavior

static/js/settings/dom.js

@@ -0,0 +1,7 @@
+// Shared DOM helpers for the Settings modules.
+// Keep this module intentionally small so panel modules do not grow their own
+// element-lookup conventions as Settings is split out of settings.js.
+
+export function byId(id) {
+  return document.getElementById(id);
+}