Let the app run under a panel page's document base

A panel page loads the app with <base> pointing at the app's server, so
addresses the app builds resolve against the document base instead of
the page location, and hash updates stay on the page's own address.
The dev server answers panel origins with CORS on the API routes too.

src/sharelink.js

@@ -529,7 +529,13 @@ export class ShareLinkManager {
     if (this._destroyed || this._initialRestorePending) return;
     const params = this._buildHashParams();
     if (!params) return;
-    history.replaceState(null, '', `#${params.toString()}`);
+    // The hash belongs to this page's address, not the document base the
+    // app may load under.
+    history.replaceState(
+      null,
+      '',
+      new URL(`#${params.toString()}`, window.location.href).href,
+    );
   }
 
   /** Build a deterministic snapshot without mutating history. */