feat: convert secret-sources into a plugin (B20)

plugins/secret-sources/src/frontend/SecretReferenceHint.tsx

@@ -0,0 +1,18 @@
+import { useTranslation } from "@termix/plugin-sdk/frontend";
+
+/** One line under a secret field: references are allowed, here is where to set them up. */
+export function SecretReferenceHint({ onManage }: { onManage: () => void }) {
+  const { t } = useTranslation();
+  return (
+    <p className="text-[10px] text-muted-foreground">
+      {t("hint")}{" "}
+      <button
+        type="button"
+        className="text-accent-brand hover:underline"
+        onClick={onManage}
+      >
+        {t("manage")}
+      </button>
+    </p>
+  );
+}