fix: harden external link navigation in the web app

- add a shared external-navigation guard at the document level
- route onboarding, markdown, settings, and node source links through one helper
- stop browser fallback code from replacing the current app page on failures

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src/components/system/ExternalNavigationManager.tsx

@@ -0,0 +1,48 @@
+"use client";
+
+import { useEffect } from "react";
+import { openExternalUrl, shouldOpenExternally } from "@/utils/openExternalUrl";
+
+export default function ExternalNavigationManager() {
+  useEffect(() => {
+    const handleDocumentClick = (event: MouseEvent) => {
+      if (event.defaultPrevented || event.button !== 0) {
+        return;
+      }
+
+      const target = event.target;
+      if (!(target instanceof Element)) {
+        return;
+      }
+
+      const anchor = target.closest("a[href]");
+      if (!(anchor instanceof HTMLAnchorElement)) {
+        return;
+      }
+
+      const rawHref = anchor.getAttribute("href");
+      if (!rawHref || rawHref.startsWith("#") || anchor.hasAttribute("download")) {
+        return;
+      }
+
+      if (!shouldOpenExternally(rawHref)) {
+        return;
+      }
+
+      event.preventDefault();
+      event.stopPropagation();
+
+      void openExternalUrl(anchor.href || rawHref).catch((error) => {
+        console.error("[external] document click open failed", error);
+        window.alert(`Unable to open this link automatically.\n\n${anchor.href || rawHref}`);
+      });
+    };
+
+    document.addEventListener("click", handleDocumentClick, true);
+    return () => {
+      document.removeEventListener("click", handleDocumentClick, true);
+    };
+  }, []);
+
+  return null;
+}